How would proportional representation work?

[JumeirahJane]

I was just looking at the election results and doing some thinking after a friend suggested I sign a petition supporting PR. I'm not so well informed on the subject, but would seem that there's a lot to be said for the current constitution which supports local representation and regional political differences. Even though national votes for the Green party totalled only 260000, Brighton Pavillion voted in their Green candidate as MP. I also note that BNP gained over half a million votes nationally (eek)which translated into 0 seats, and UKIP 850,000 votes = 0 seats.

How EXACTLY would PR work? Do I still vote for my own MP working in my constituency? If so, where do the extra MPs come from? Would PR mean that the Green party would somehow be denied their seat on the grounds of national votes? Without wishing to get into discussion over the merits of particular parties, I am genuinely interested if someone can explain or post a link.