a whammy pedal for example a "digitech whammy" pedal does what the whammy bar on a guitar does, hightens and lowers the note, as you push the pedal down it lowers the note just like pushing the whammy bar down does.

a tremolo pedal increases and lowers the volume in set pulses in relation to a time period usually set as a parameter on the pedal. this creates a stuttering effect as if you turned the volume knob on the guitar up and down really fast.

a good piece of advise is forget about calling your whammy bar a tremolo arm, just completely disregard it as part of you guitar vocabulary (unless your buying spare parts or something over the net, cause the word comes in useful then, lol), it avoids a lot of confusion

hope that helps