A colon ( : ) is used somewhat like the word "which" and a form of "to be" such as "are", "is", "was", and other such words. 

Example:
He passed by a lavish room filled with opulent-looking knick-knacks: a sparkly cat; fancy rugs; and beautiful, silk tapestries depicting everyday Greek life. 

The colon indicates that the following things are the aforementioned opulent-looking knick-knacks. Those fancy-looking commas with periods on top of them are called semicolons which is explained in literally two sentences. 

Here's an another example:
The new goal was set: get the escapees rescued before dawn.

The colon in this sentence indicates what the new goal is.

Now let's look at a semicolon ( ; ).

A semicolon is used to connect two sentences together in place of a conjunction. A conjunction is a word that links two ideas together such as the words "for", "and", "nor" etc. A teacher of mine calls them FANBOYS. 

F - for
A - and
N - nor
B - but
O - or
Y - yet
S - so

If it helps to remember it that way, then remember it that way. I will probably remember it this way for the rest of my life. I'm just sayin'. 

Here are some examples of semicolons used properly:

1. My neck had begun to ache; I'd been looking up for so long. 

2. I looked to the left; the other option was simply unbearable. 

3. We'd begun the research; the board had approved it. 

You use a semicolon when you're connecting two sentences together that are related.

To show, you wouldn't put these two sentences together:

My neck ached. The dogs were fine.

Your neck aching has nothing to do with the dogs being fine. Those are both fine sentences, but they're not at all related. It all really depends on context, though, so if for some reason your neck aching did have something to do with the dogs being fine then it'd be correct. You get my point though, right?

I love this new WattPad feature.

The memes are strong with this one.

If any of you are confused as to how to use a semicolon, then just tell me and I'll try to explain it better!