Showing vs. telling and description go hand in hand, so I'm putting them in the same chapter.

Good writers show. They don't (just) tell.

Briefly, telling means you're telling someone something. Showing means you show it to them through sights, sounds, smells, and other sensation and let them figure out what it means. Showing is generally more effective, BUT:

Good writing has a balance of showing and telling. Do not tell everything and do not show everything. Find the middle ground. However, most novice writers automatically tell more than show, so this thread will focus on how to show things rather than tell. Still, remember that story is made up of both showing and telling.

Generally speaking, showing uses more details and elaboration. It takes more time to accomplish, but it is more effective in getting your reader engaged in the story.

Example 1
Telling: "Phil was angry."
Showing: "His hands curled into tight fists at his sides. Veins popped out of his arms as his muscles tensed."

When you state an emotion, you are telling. When you describe the physical actions associated with that emotion without outright stating the emotion, you are showing.

Example 2
Telling: "She was really nerdy."
Showing: "Without prodding, she pulled out a biology textbook--I almost never saw her without that thing--and flipped to a page in the middle. She pushed the bridge of her glasses farther up her nose as her brows furrowed. I knew she wouldn't leave that hunched position until dinner."

When you state a character trait, you are telling. When you describe how that person acts, you're showing.

Now, since I'm lazy, I'll tell you to go to the site in the External Link for more showing vs. telling examples. It goes into a lot of detail and it's an excellent resource that I learned a lot from!

Adverbs
Adverbs tell rather than show.

Don't tell us he ran quickly (the adverb would be "quickly"). SHOW us his racing breath, the numbness in his legs, the burning in his throat.

Don't tell us the girl screamed loudly. Show it through the reactions of others--covering their ears, wincing, gritting their teeth together.

A good description is more like a mini mystery novel. You have to give the readers the clues and let them put them together to paint a picture. A mystery novel is powerful because the reader has to figure out what's happening for themselves. If the author straight-out told them who the murderer was, that wouldn't make for an interesting read. SHOW us. Don't tell us.

Another tip for description: Describe the most important things in your story the most. You don't have to describe every knothole in the maplewood table with four legs. If it's just a table, just say "table". Unless those details are vital to our understanding of the plot or character, leave them out.

Likewise, your MCs love interest is probably important. She/He gets more vivid description than the chair he's sitting in. If your MC is a witch and gets her first wand, the author would want to describe that wand like there's no tomorrow.

How to immerse your description in your story
Do NOT info-dump paragraphs and paragraphs describing the house your MC visits. To make an environment more engaging, do NOT describe it. Your MC needs to REACT to it.

I'll use an excerpt from SuperHero for an example. A lot of people said how they loved the description here, so I'm using it. :P

“Done!” I slammed the stub of a pencil down on the table and jumped from my chair so quickly that it toppled over. The breeze I caused kicked up a dust cloud that reduced everyone in the room into a coughing fit, and the mice watching us from their hole in the corner started squeaking in protest.
...
“You're done already?” A middle-aged man named Leonardo huffed at me. One of the books propping the faulty table leg up started to slip, so Leonardo kicked it back in place before the entire table collapsed.
...
The cramped area was not the grandest place for physicists to design new airships for the Army. It housed a single table with five chairs crammed around it. The men and women working around it had to use the little floor space to fan out their drawings and calculations, and the moment I had gotten up from my seat, it became another storage surface for papers and folders. A single, bare light bulb hanging from the ceiling provided the only illumination, and I feared that a few more years of this would strain my eyes into blindness.

Notice how I didn't just throw all the imagery of the room at the readers all at once (the ... means that there was a chunk of story I skipped, so these three paragraphs are spaced out through the chapter). The characters are interacting with their environment, and that's the best way for the readers to learn about it. It'll stick in their heads a lot longer if the environment is part of the story. Same goes for school work--if you integrate the lessons into your daily life, you'll be more likely to remember them than if your teacher just lectured on them for an hour and sent you off.

Insert imagery naturally into the story. If your character says something, make them do something with the setting, like Leonardo did kicking the book back into place. It shows not only that the table is in really bad shape, but also that they're probably poor for not replacing it with a good table, and it gives insight into Leonardo's aggressive personality. Three for one! :) Now if you just told us that the table was bad and had to be propped up on a book, you wouldn't get nearly as many details in with that statement.

The third paragraph borderlines on telling, but it still has the characters reacting to the environment because it says how the MC's spot was immediately taken over by the others' things. She didn't just tell us that there was a single light bulb hanging from the ceiling, she also reacted to it by saying it strained her eyes, and those reactions make it more interesting to read.

Rather than describing all this and making your reader just skim over it, or worse, skip it altogether (don't lie, you know when you've seen three pages of description in a book, you literally flipped to the first piece of dialogue after it.), immerse actions and dialogue in between them. Vital things that your readers can't miss. That will force them to read through the descriptions. :)

You have to hide your descriptions in the story. So there's this big, ol' house. The MC could be driving up the pathway, but it's unpaved, and the ride is really bouncy. She doesn't have to tell us it's unpaved. We can figure it out when she comments how rough the ride is. She could be walking around the front yard, and she'll get scratched from the thorny weeds. Rather than telling us the grounds were unkempt, we can see it for ourselves. Don't be afraid to make your readers think a little and make their own judgements.

Dialogue and internal thoughts really help, too! Those are also a form of reaction. :)

So the three things to remember when making your characters react to their environment:
1. action
2. dialogue
3. internal thoughts

From the awesome Jelsa who taught me about this, just remember EAT:
E = expression (facial).
A = action (body).
T = thoughts (mind).

EXERCISES!
Convert the following statements from telling into showing, and share your answers in the comments:
1. I punched him.
2. The little boy was tired.
3. He was annoying.
4. That sunset looked beautiful.

Upon request, here's a short blurb of when to show and when to tell:

The majority of your writing should be showing. Show the characters' emotions, thoughts, and actions without telling them. However, showing takes up a lot more time and space, and it slows down the prose. Sometimes saying "He punched me in the face" is preferable to "A bead of sweat dripped down the contours of his cheek bones as he balled his hand into a fist, reeled back, and let it fly into my face. His sharp knuckles clawed into my jaw, and my head cracked to the side."

If the scene you're writing is really fast, a long, drawn out description of what happens may not be the best choice because it slows down the prose. That's your call as the writer, and I can't really show you how to do it. You have to practice a lot until you can effectively "feel out" the pacing of the story. (I personally stink at pacing and make everything too quick, so I can't give any advicing on that, unfortunately.)

That gets a bit technical, though, so here's an easier way to look at it:

Describe the most important things the most.

If the character is sitting at a table, but she's about to get beamed up by aliens, the focus should be on the aliens, not on what type of wood the table is made out of. If, however, knowing what type of wood the table is made out of IS necessary to the reader's understanding of the story, then you should mention it. If it's not important, it's usually best to leave it out. Keep your writing as "clutter-free" as you can. If the story makes sense without a word/sentence/paragraph/scene in it, you can usually take it out. There's no need to saturate the writing with hundreds of adjectives and flowery, poetic prose. Keep it as simple as you can, and that's what gives your words power. That way, your readers will focus on what's important and won't get distracted by extraneous details.

When you have scenes or actions that aren't important to the story, such as the character waking up in the morning and going to brush his teeth, you don't want to drone on and on about the toughness of the bristles and the minty coolness of his toothpaste. Just say "he brushed his teeth".

If several months pass in the story where nothing really important happens, it's all right to skim over that time with a couple sentences of telling. No need to spend half the book talking about your character going to school and doing homework if the real story doesn't pick up until her sister gets married to a psychopath a year later.