Bitcoin, or "Bitcoin wallet", is the name of the client software you install. After you run the software, it will generate a Bitcoin address for you. You can give this address to other people, and they will be able to send you Bitcoins. Under "receive coins" in the Bitcoin software, you can generate more new addresses, so that you can give different address to different people, and all the payments will still come to you. This is useful if you want easily determine who sent you money.
Once you run the software, it must download all transactions that have ever taken place, so that Bitcoin knows how much money every user of Bitcoin has. This can take many hours, or even a day or two on a bad internet connection. As a new user, you will only be able to see money that has been sent to you after all transaction blocks have been downloaded.
After you have received money, you can then make a payment to another user by sending coins to one of their Bitcoin addresses.
It is important that you make a backup of the wallet file, as only the computer you install Bitcoin on has the information you need to send coins - if your hard drive crashes, you will lose all your money, because you no longer will be able to send it to anyone. Please look at the wiki to learn how to make a backup of your wallet.dat file, which contains all your addresses.
How do you get bitcoins? You trade something for them, either money, services, or merchandise. There are currency exchanges where you can simply buy bitcoins on the internet. There also may be users near you that will sell Bitcoins in person, or may pay you Bitcoins for something. Sometimes there are people here that will even send you small amounts for free so you can see how Bitcoin works, simply give them your Bitcoin address.
You should become a super-expert in all aspects of Bitcoin before considering using it on any website or internet service.
thank u very much sir for info i hve learnd already btw my block has been downloaded but only 50% is this for first time ?