Which symmetric key algorithm does SSL use

You can download the Twit.tv Security Now podcasts to get yourself familiar with the concepts. That or get them from GRC.COM/Securitynow. Start from #1 and move forward until you believe you've got enough. That or you can get one of the CompTIA Security+ certification books and read to get background.

That said, most of what you are discussing is what is known as PKI. Public Key Infrastructure is a way of encrypting info in which you create a key pair. One of the keys is called public and you publish it. Anyone can encrypt data using your public key and send it to you. The other key is called private and only you must know it. Only the private key can decrypt the data. Similarly, you can encrypt using the private key, send the data out and the receiver gets your published public key and only it can decrypt it.

RSA and DSA are two encryption forms. Don't worry about the difference. You basically need to sign up with a firm like Verisign who will help you with an account and you create the key pair. They publish the public key and you keep the private key.

PGP is a program which uses similar technology. You install the program and create a key pair. Send the public to PGP's server. From then on, when you send mail, it can either encrypt using your private key and the receiver needs the public key to decrypt or you can create a hash - a short numerical representation of the email - which is then attached to the bottom and created using your private key. The receiver reads the email and using software, creates a hash using your public key and compares it to the signature. If they match, nothing was altered and it proves you sent it.

As far as SSH, I'll leave someone else to explain protocols.

The Public Key is one way. You can not decrypt the communication with it. You need the private part of the key pair to do the decryption.