Slip knots are very useful in knitting and crochet. A slip knot is a quick way of securing a loop of yarn to a needle or hook, and is one of those knots that gets tighter the more it's pulled. (Here's some knot trivia: the slip knot and noose knot are very similar and are often confused.)
While knitting is about rows, crochet is about chains. (Well, crochet is about rows too, but you'll hear "chain" a lot.) Chains can be arranged in rows, or clumps, or circles. Making a chain is the base technique for most crochet.