Mathematica notebooks support a variety of input and output styles. You can write input using the characters from the standard keyboard. Alternately, you can write input in more familiar mathematical notation using the front-end palettes or keyboard shortcuts. Learn more in this "How to" screencast.
Mathematica allows Greek letters to be integrated into symbol names, strings, graphics, and text. You can input Greek letters by using palettes or keyboard shortcuts. Learn more in this "How to" screencast.