There are two basic techniques for encrypting information: symmetric encryption (also called secret key encryption) and asymmetric encryption (also called public key encryption.
Symmetric encryption is the oldest and best-known technique. A secret key, which can be a number, a word, or just a string of random letters, is applied to the text of a message to change the content in a particular way.
This might be as simple as shifting each letter by a number of places in the alphabet. As long as both sender and recipient know the secret key, they can encrypt and decrypt all messages that use this key.
The problem with secret keys is exchanging them over the Internet or a large network while preventing them from falling into the wrong hands. Anyone who knows the secret key can decrypt the message.
One answer is asymmetric encryption, in which there are two related keys--a key pair. A public key is made freely available to anyone who might want to send you a message. A second, private key is kept secret, so that only you know it.
There is no way to directly compare them. We would point out the only thing we can say is secure is one time truly random pad based XOR stream cipher (which is symmetric), however key exchange is a major problem.
We can also say that we can, in theory, break RSA and El Gammel (the two main asymmetric) algorithms) with quantum computing, we just haven't built the device to actually do it, until...