well lets say hold your first finger g 3rd fret take your 3rd finger and tap and hold ot on the g 5th fret
you grab a hammer and a hand full of nails... line up the nail on your fretboard... then hit it as hard as you can into the fretboard...then you repeat for as many times as you want... that is a hammer technique....

but seriously... there are some videos on the web and some lessons on this sight... try um out
And so you dont feel inclined to comeback and make another thread, a pull-off is the opposite, where you play a string and then pull your finger off of the fret real quick as to retain the sound.
Just couldnt bring myself to do it. I was going to suggest that this might be better posted in the Pit since those folks are always so helpful. Then I saw the register date and couldnt bring myself to it. Hammers and pulls are quite effective techniques that are fairly simple to accomplish check out the thread that jimtaka posted. This will give you the basics. Hang in there...