eBay owns Paypal, so if you have an eBay account, there will be links and banners all over your control panel and other places that will basically hold your hand thru the process of signing up.
you put in your bank account info, paypal makes a small deposit of a few cents into your account, you then tell them how much it was. this verifies its your account.
this bank account is then directly linked to paypal and your email name.
if anyone ever needs to send you $$ they can go to the paypal site, put in your email name and then send you $$ in a variety of ways. credit card, bank transfer etc...
it then shows right up in your account
many, MANY stores outside ebay also use paypal as a payment source. this is a great way to pay because the store never sees ANY of your account information. all the transaction is handled thru paypal so you are very safe.