Need some help encrypting drive with TrueCrypt

So I decided to encrypt my whole drive with TrueCrypt. I made a random password of 30 characters and chose AES (AES-Twofish-Serpent was just too slow) as my encryption.

I have some question though, and could use some help. First of all, I am now at TrueCrypt's "wipe" screen. What do they mean with this? It says existing data will not be lost, but AFAIK usimng 3,7 or 35-pass rewriting of data is only used to wipe(delete) things ?

Another question, once I have encrypted the disk, any new data written to it (By Windows, an program or just plain data) will also be written to it encrypted, right ?

I am on a fresh new install(Minus the drivers and programs I just installed) of Windows 7.

If anyone could clear this up I would be very thankful.

Thanks in Advance!