Raspberries are very hearty, you can pretty much leave them alone and you will still get a decent crop of fruit. However, when left alone a raspberry bush will quickly expand and take over the surrounding area. To keep your raspberry patch under control, and to ensure a good harvest of fruit, it’s best to prune your raspberries every year.
The most important thing to keep in mind is that raspberry canes only bear fruit in their second year after which time the cane dies. So the basic idea is to 1) Prune the old canes (the ones with fruit the previous year) right to the ground; 2) thin out the canes which grew during the previous season.
Here is a great Youtube video which explains everything you need to know about pruning your raspberries: