One of the most important tasks of Network Security is to protect precious private Data from eavesdroppers. Loss of private data to wrong hands is catastrophic to any business. Cryptography is a branch of Computer Science / Mathematics, which deals with Data Protection, from eavesdropping. Cryptography is the art and science of making data impossible to read for eavesdroppers.
Encryption is the process of scrambling Clear-Text (also called as Plain-Text, which is any readable data in any format) using any Encryption Algorithm to make Cipher-Text. Once the Data is scrambled, it is called as Cipher-Text, which is a just some meaningless text.
Note: Scrambling is a term which is used to mention convertion of the Clear-Text to unreadable meaningless text, Cipher-Text.
Encryption algorithms use "Keys" while encrypting the data. An Encryption Key is secret value, which is used as an input to the Encryption algorithm along with the Clear-Text data when Clear-Text is converted to Cipher-Text. Keys determine the output of an Encryption algorithm. You may think an encryption Key as a "Password".
Once the Data is properly encrypted, it is impossible for an eavesdropper to read it, since the original data is completely scrambled and the eavesdropper doesn't have the original encryption Key.
Decryption is the Process of converting back the Cipher-Text to original Clear-Text.
Without the original Key, it is theoretically impossible to decrypt the Cipher-Text to original Clear-Text.