What is Beamer? Simply put, it is a LaTeX class that allows you to create a versatile and interactive PDF slide presentation. Though similar to other packages, like Prosper, is has an advantage in that it works directly with pdflatex and dvips.
Beamer also allows for the printing of handouts: either in the form of slides-on-paper or as an article, with frame titles used as corresponding paragraph titles. This feature allows for the easy creation of simple-to-follow lecture notes.