A firewall is usually a combination of hardware and software that insures that only authorized entry into a system is permitted. Firewalls that work very well are difficult to create, and require consistent monitoring for effectiveness. None are foolproof, and they can be a real pain for users. The creation of a firewall is a highly sophisticated process, usually used only by large companies with dedicated lines and computers. Most companies do not need the ultra-high security provided by firewalls.
Encryption involves the encoding of material such that the information is relatively inaccessible.
The encrypted, unreadable message is then transmitted across the Internet. The recipient then uses decryption software compatible with the sender's encryption software, using the message's key. The material is then reprocessed back into its original form. A single key that both encrypts and decrypts is called a one-key algorithm or system. A system that requires two keys typically uses a public key to encrypt the message, and a private key to decrypt the message.