So many key functions...

'CLICK' to create particles at the mouse

'A/D' or 'LEFT and RIGHT' arrows to toggle particle type (shown at the bottom of the screen)

'C' to toggle custom particle mode, which uses the values set in edit mode.

'T' to toggle edit mode (More below...)

B' to change background

'M' to toggle mode (user mode or auto mode, basically auto mode creates particles without you having to click :P )

'P' to pause all particles

'SPACE' for awesome slow-mo effect!

'V' for volume slider!

IN edit mode, use the mouse to hover over options, and then A/D or LEFT and RIGHT arrows to change the values. The buttons at the bottom do what they say (you can copy/paste other peoples codes to see what they have created), you can also press 'R' to randomize all values.