Cryptography is a science of secure communication between hosts. It includes securing the message which can be only be read by person with the right code. The process is called encryption. And the process in which secure message is decoded is called decryption.
There are different types of cryptography - depending on the type of methods being used in the communication. Three popular types of cryptography techniques are - public key, private key and symmetric key cryptography. In each case of public key cryptography, the content is secured with PGP key and you need the same key to read the message.
The public key content doesn't have to secret and can be shared with others. Such cryptography is only used for the content that requires security from some sources that doesn't allow open communication.
In case of private key cryptography the content is secured and the key is only shared in between two or more private parties.
In case of symmetric cryptography, the same key is used for the encryption and decryption. This was used in old method for the encryption. This is obsolete method for secure communication.
There is also another improvement in the cryptography industry known as "Steganography". In this method of secure communication the data is hidden within another content. For example simple nature image could have mp3 file hidden inside. Such method of encryption is very popular and harder to break for those who are into security analysis.
DES is an old method of data encryption. It was similar to the symmetric data encryption. In such type of data security the same key was used for encryption and decryption.
RSA is another cryptosystem which is rather slow algorithm that keeps public and private key separate. Each type of key has different mode for decryption. Public key may be used for encryption but the decryption key is made secret. Most of the programming languages such as R, Python, Java, Ruby allows encryption using RSA, SHA and MD5.
Cryptography is very vast field with new improvements being made every year. You can learn a lot more by following the Cryptography journals and IEEE security journals.