Parse Let's you control what the user typed in at the command line.
Form Get access to the all answers right before they're displayed.
Exec Mess with the two variables being calculated.
Expr Gives you control over the equation numbers right before they're printed on the graph screen when graphing.
Home Run your own routine right before the calculator returns to the homescreen.
CmdTok Executed right before any TI-BASIC functions (DelVar, Asm, Bin, etc.) are run.
Key Probably the most popular, have the calculator run your program every time _Getkey is called.
Grf Implement your own routine right before graphing functions.