You know that guy who lives down the road and always parks his car across the zebra crossing? The car which has a lovely vinyl sticker of a soft-pornographic silhouette on its bumper? Well it doesn’t matter whether you know him or not, the second clause in each of those previous sentences were relative clauses: clauses which refer back to a noun which was mentioned in the previous clause, i.e. ‘..who lives down the road…’ (the guy), ‘which has a lovely sticker…’ (the car).
We use them in English and German all the time; if you don’t believe me, try to keep an ear out next time you have a conversation with someone for the number of times you or your friend refer to ‘the something who/which…’. It’s a extremely useful construction and, if you hadn’t already guessed, it’s got a few important rules in German in order to get it right.