ok so i thought i will try and explain breifly about each.
EBAY:
an online action site to buy/sell items from accross the world at good low prices, great for getting bargains
PAYPAL:
like the middle man between your personal bank account and buying items from ebay mainly. basically you register your details on the paypal site, register your bank account and credit/debit card their also. then whenever you buy something from ebay or whahtever site accepts paypal, you get put through the paypal checkout, then type in your password/email address when asked to do so for your paypal account, and the payment deducts from your personal accounts balance.
it all sounds complex but its actually one of the easiest ways of buying online, also when you sell items on ebay, the money you recieve comes to the paypal account, the you have to log in after recieving payment/s and click wihdraw funds to your personal account.
NOTE:MUST BE 18 OR OVER TO MAKE BOTH OF THESE EBAY AND PAYPAL.