Few Questions on Bitcoin

[AntiOps]

Sorry if its pretty obvious questions, I'm a newb here so bear with me. I have two questions:

1. When you download the MultiBit client you get a pre-assigned address, which you do not have to create. It's not protected by a password by default so what actually makes this my personal address if its not protected by a password?

2. Whats the difference between Bitcoin-QT and MultiBit? I know MultiBit is micro compared to the first. Why does Bitcoin-QT have to be massive and not MultiBit?

Thanks in advance

[vaio127]

On first run, Multibit will generate a new address that is entirely localized on your computer.  If you decide not to have it encrypt your addresses, you'll still be the only one capable of using it, but there's a higher probability of it being stolen at some point.

Bitcoin-qt downloads the entire blockchain (currently about 9.5 gb) whereas Multibit does not.

[AntiOps]

vaio127 thanks for the reply,

By encrypting you mean put a password on it right? And if theres no password, I'm guessing there is some other unique data that verifies the ownership of the address that is in a file somewhere located only on the clients computer?

Edit: multibit.wallet file contains the unique data, right..?

And why does Bitcoin-QT download the entire blockchain if its not necessary to run, more to the point why would anyone download qt if MultiBit is quicker and smaller?

Sorry, I'm just trying to understand Bitcoin as a whole including the technical side.

[vaio127]

The 'unique data' is the private key for the address.  When a client generates a new address, it first chooses a random 256-bit number (the private key), and then calculates the public key, which is the address that is used when sending or receiving funds.  Only the person who has the private key which corresponds to the address will be able to spend with it.

The Bitcoin-qt downloads the entire blockchain because it also acts as a node, passing information around and confirming transactions for the entire network.  Multibit doesn't do this and only processes information that is relevant to one of your addresses.

[AntiOps]

Perfect, now I understand it more clearly. Cheers!