I've just joined Google+ and received in my email straight away ' people I may know' with an add..is

This is an automatic search facebook does... it offers you list of friends or their friends or their address book contacts as people you may know.... esp. If you already added friends... so the facebook social media tries to get your circle bigger and bigger by using your own information ..like your synchronised address book in your phone... but the policy encourage not to add anyone you don't personally know.... just people you met actually...
And for difference between people you may know and requests... go to your facebook page... and click on your profile... click on side menu ..notifications.... it will explain it more thoroughly ..hope it helps.. regards..