Similar to fonts for textual content, there are formatting styles or fonts that can be used on math expressions in LaTeX. For example, the set of real numbers R is typically written with what is called a blackboard bold font.

The formatting styles for math in LaTeX are:

mathnormal: This is the default for math expressions. The letters are italicized a bit. No need to apply it explicitly.
mathrm: Non-italicized version of normal.
mathit: Italicized like normal, but suits words better.
mathbf: Bold. Use for vectors and so on.
mathsf: Sans serif.
mathtt: Fixed-width.
mathfrak: Fraktur. Used for lie algebras.
mathcal: Calligraphic.
mathbb: Used for real sets.
mathscr: Script.

A table of these fonts applied on symbols can be seen here.