Okay, so we all know the famous Dream Demon himself, Bill Cipher, but do we really know his true intentions and what he actually is?

The evidence that I use mainly belongs to: http://triangulum.wikia.com/wiki/Triangulum_Information

So, what really is Bill? Well, we know that he's a "being of pure energy with no weakness" and has a triangular shape, so we can infer that Bill is not a Demon, but a Triangulum. Triangulums are creatures who maintain a triangular shape and are made up of pure energy. Like a human fingerprint or a small snowflake, not one is the same as the other.

More evidence pointing to Bill being a Triangulum is that, when he speaks, he glows.
Since sounds are made up of vibrations, Triangulums vibrate at certain frequencies to project voices or other sounds. When doing this, they light up.

Along with these defining features, all Triangulums have one giant eye near their point. Sound familiar?

Triangulums also come in different levels of power.
While some of them may be beginners, being only able to do certain things like read minds or levitation, Bill is obviously a highly advanced breed of Triangulum, being able to manipulate minds, do pyrokinesis, enter minds, and many other things.

Triangulums are the "protectors of the Earth". They help humans to understand and warn them about things to come. Bill doesn't seem to do this. But, in a way, he is.

All this time, Bill had been telling the Pines Twins to "leave before they see something they'd regret". He was warning them about the dangers of the paranormal and, whenever he tried to get the journal from the Pines, he was trying to keep them from being any more interested than they already were.

A prime example of this would be in the episode, "Sock Opera". Everyone knows the famous scene where Bill possesses Dipper's body and destroys the laptop. He did this to keep Dipper from exploiting any more mysteries about the town.

While he is referred to as a demonic entity by Gideon, as Triangulums often are, he really isn't. Bill is an elemental creature of sorts, but, being called a demon for so long, he probably assumes that he is one. Alex Hirche really did his research!

In conclusion, we can rightfully assume that Bill Cipher is not a Demon, but an entity by the name of a Triangulum.