Oil pressure switch, what does it do?

[renz]

Hello,
First post so please excuse me if I do something dumb.
About two months ago my oil pressure switch broke. Thanks to some other posts in these forums I managed to replace it myself. Being an absolute newbie I was delighted, however yesterday the car started leaking oil again. I wiped clean the oil pressure switch and oil filter which is just below it, and started the engine. It is seeping from the pressure switch again. I don't even know what the oil pressure switch does so was wondering if somebody would be kind enough to tell me, and also advise me how common these things are to go? did i do it up to tight etc...
Car = Audi A4 96-01 model 1.6 Petrol Manual, I can't be sure of the Reg as I inherited the car and we cant find the log book, and it has a private plate on it.