Watch the Trill online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
Combine repeating, rapid-fire hammer-ons/pull-offs, and you have what is known as trills or trilling. Whether its part of a riff or lick or simply the moment where you put your pick hand to your ear signaling your audience to "make some noise", trills are just plain cool.
Looking for inspiration? My first guitar hero was Steve Howe of Yes. Trills are an important part of any prog rock guy's trick bag, and a few seconds past the 7:00 mark in "Roundabout" is a big reason why.