Plain ASCII is normally sufficient for almost all note taking. Exceptions include scientific notes, which often require mathematical symbols and the occasional formula. LaTeX110 is widely used to typeset scientific documents. Org mode supports embedding LaTeX code into its files, because many academics are used to writing and reading LaTeX source code, and because it can be readily processed to produce pretty output for a number of export back-ends.
LaTeX is a macro system based on Donald E. Knuth’s TeX system. Many of the features described here as “LaTeX” are really from TeX, but for simplicity I am blurring this distinction.