Encryption

Encryption is the art of obfuscating information so that a third party cannot read its contents.

Meet Alice. (person A)
Meet Bob.  (person B).

Alice likes Bob and wants to write Bob a loveletter.

Meet Eve. (the person that eavesdrops).
Meet Mallory (the person that likes to destroy things).

Eve and Mallory want to know what Alice writes to Bob. Mallory wants to go even further and anger Alice and Bob.

There is different types of encryption, symmetric and assymetric. One requires a key that is well known on both ends between Alice and Bob. If Bob doesn't have the secret key, he and Alice can use public encryption to work out a new key in a secure fashion. Supposedly Eve cannot determine the new key. But Mallory can still get in the way by disrupting communication. At this point encryption won't help either and noone is any smarter about what the key is that Alice holds, not Bob, not Eve and especially not Mallory.

The encrypted text in encryption is called ciphertext. In plain text it's called plaintext.