Bitcoin is a software-based online payment system described by Satoshi Nakamoto in 2008 and introduced as open-source software in 2009. more about Bitcoins at wikipedia
Start here. If you are going to use it only for fun choose WEB wallet.
Microwallet is a place where all of your coins accumulate. Once they reach a withdrawal limit they will be sent to your real wallet address.
Most of the websites get their profit by showing ads. And most of them are able to detect the ad-blocking plugins. Be fair, we don't want them bankrupt, right?
A satoshi is currently the smallest unit of the Bitcoin currency (1/100,000,000 BTC or 0.00000001 BTC). The unit has been named "satoshi" in collective homage to the founder Bitcoin, Satoshi Nakamoto.