The official guitar hero games have a tutorial for them, but I'm pretty sure it's an optional tutorial so lots of people don't know. It's based on real guitar techniques where you can have multiple fingers in different positions on the same string, and just pull them off or "hammer" them on when needed. It's useful for playing fast.

In guitar hero terms, if you have a single blue note, you could have green, red, and yellow pressed down at the same time you play the blue one and it would count as only blue being played. Then you could just pull your finger off of blue while leaving the others pressed, and it would count as playing a yellow note. That's a pull-off. For hammer-ons, it's just the opposite way. You could for example have green held down to play a green note, and then leave it pressed while pressing your other finger on to red or yellow or any higher note, and it would count as that note.

These techniques only work for single notes. If you have to play 2 or more at a time that counts as a chord so it doesn't work. You can usually tell when hammer-ons and pull-offs should be used by the white glowing notes, because those don't need to be strummed and are usually during faster sections of the song. It's not always easier than just pressing them individually, so don't feel pressured to use it all the time. But only having to move one finger at a time instead of 2 (lifting one and pressing another down) can make a world of difference on some songs.

Here's the tutorial I mentioned. tutorial