Chords are button presses that have 2 or more notes. Usually when experts refer to cords, they're 4 to 6 button chords that the player must use both hands to hit, and the chords keep coming up every 8th note.
Example:
http://video.google.com/videoplay?do...47&q=otaku1313
Jackhammers are when you have to press a single button or a chord very fast (like in Classic 8 EX, also a lot of the older songs have them), usually at the speed of 16th notes. The button is the same button in every note in the jackhammer, but you have to hit it very fast.
Example:
http://video.google.com/videoplay?do...40&q=otaku1313