Newbi wallet question

Hi forum member, i just discovered bitcoin a week ago. i know this question has been probably asked many times before.

i downloaded bitcoin wallet from bitcoin.org and i have synchronized it with network and everything( took me 2 days). There is just a few things i don't understand.

how do you backup your wallet? i know when i go to the wallet menu it creates wallet.dat file but the thing i dont understant is how to use that file. so i can just save that file and if i choose to delete the wallet from computer completely and install it again lets say on a different computer, can i just take the wallet.dat file that i have backed up and paste in into the directory where i have bitcoin wallet installed or how does it work? ( i looked through the directory and i didnt find any wallet.dat ?!
So i dont understand how to backup my wallet.

also i am not sure about the "Encrypt wallet". when i click it it askes me to create a password which i did but how does it really apply, since when i open wallet program it doesnt ask me for password.

Also , in the wallet program menu there Debug window option which opens a window with a bunch of numbers, what is that all about.

and one more thing... at the bottom right hand side after encrypting my wallet there is a little icon that says "wallet is encrypted and currently locked". What does that mean? Does that mean?

Thank you all.