A riff is a short sequence of notes or chords played repeatedly. Simply put, it's THE guitar melody, THE thing that catches your ear and makes you want to move. Rock songs are more or less made of riffs put together, and almost all of them have a "main riff" that is usually played from the beginning of the song, without the singer interfering. So, if we mention "Smells Like Teen Spirit" by Nirvana, you can probably sing the main riff. Or maybe "Seven Nation Army", by the White Stripes. Touuum… Toum toum toum toum, touuuumm… There! You already know some riffs by heart.
And there are plenty of mythical riffs. For example, "Smoke on The Water" by Deep Purple has become the favorite teaching tool of guitar teachers. The "Satisfaction" riff by The Rolling Stones causes anyone over 50 an air-guitar episode. Some riffs have even made history, such as those of "Reign in Blood" by Slayer, and "Master of Puppets" by Metallica, which are famous for laying the foundations of trash metal in the 80’s.
While the riff is vital, the quality of a "good riff" is still a matter of taste. In 2014, the BBC asked its listeners to vote for their favorite riff in rock history. The result is a rather subjective ranking of fifty songs. The best one? "Whole Lotta Love" by Led Zeppelin. You be the judge!