TrueType
TrueType is one of the two shapes a font file comes in, and the one most fonts are delivered as.
In more detail
A font has to be written down as a file before a machine can set anything in it. TrueType is one way of doing that: the outline of each letter, the room to leave on either side of it, and a set of instructions for keeping the letter crisp when it is drawn very small.
The file ends in `.ttf`, and a machine you buy comes with dozens of them already in place. It is also what a font bought from a foundry usually arrives as.
What it does on a real document
A document carries the fonts it is set in inside itself, so it opens looking the same on a machine that has never had them. Only the letters actually used are carried, which is what keeps the file small even when the font is large.
A request hands the file over beside its description, and every piece of text names it by the name the request gave it.
Where it comes from, and why
It was drawn up in the late nineteen eighties as an answer to a format that had to be licensed, and it was given away with the two systems of the day, which is how it came to be everywhere.
It was later folded into a wider format that also takes the older outlines, which is why a modern font is often both at once.