DES is a public-key encryption scheme. Triple DES involves three encryption functions on the data to be encrypted/decrypted and is considered much harder to break than plain DES.

You can send any data you want over SSL, including that which is encrypted with other encryption schemes like DES. SSL uses stream encryption, which works differently than public-key encryption.

There are plenty of resources on the web which explain encryption in detail; it is an interesting, if not a bit mathy, subject.