Tablature:
E|-------------|
B|-------------|
G|---7--h--9---|
D|-------------|
A|-------------|
E|-------------|

Similar to the bend there is no way to express the proper tablature version of a hammer-on online. On the Internet you will see it noted by the letter 'h'. The 'h' will always appear right before the note that should be hammered-on. In printed tablature the hammer-on is noted with an arc that is above the two notes.

A hammer-on involves two or more notes however you only have to pick the string once. Using the example from above; here is how it works. Your 1st finger presses down on the 7th fret, you pick the string, your 3rd finger hammers down on the 7th fret while you continue to press down on the 5th fret with your first finger. What is hammer down? Essentialy you are simply placing your 3rd on the next note but hits the fret, as if you were tapping it. The end result should be the sound of the 5th fret followed by the sound of the 7th fret all done in one motion, and one pick of the string.

Play a hammer on:
1. Place your first finger on the 7th fret of the 3rd string.
2. Pick the third string.
3. Continue to hold down the 7th fret; slam your 3rd finger on to the 9th fret.

You don't really have to slam your finger down I'm just trying to explain it...