Watch the The Mighty Riff online guitar lesson by Angus Clark from Focus On: British Metal
Riffs are at the heart of this style. A riff should pay off in a melodic way, it should have a rhythmic hook and for bonus points it should include something tricky. The best way to get inside what a great riff is is to learn some examples. The exercises in this segment are mostly about playing melodic ideas with chords cleanly, and playing rhythmic ideas with conviction. The riff is the make or break it of the song.