PGP  ("Pretty Good Privacy"), which was invented in 1991 by Phil Zimmermann,  is perhaps the most widespread encryption system in the world.

• Symmetric encryption is the 'classic' form of encryption that uses a simple password. This  means that, for a receiver of an encrypted message to be able to decrypt  it, the sender must first send him the password. This is extremely  risky and it is the reason why symmetric encryption is not ideal for  encrypted communication. It is however ideal for encrypting data (e.g. text, files) for oneself.

• PGP Private/Public key encryption (asymmetric encryption): PGP asymmetric encryption solves the problems that symmetric encryption poses for encrypted communications.
  Each individual now has two mathematically related keys (a "key pair"): a Private Key and a Public Key. These keys are mathematically related in the sense that the Public key is uniquely associated with the Private key of that key pair in such a way that, if one knows the Private key,  it's  extremely easy to derive the Public key from it. However, if one  knows the Public key, it's 'kind of impossible' to derive the Private  key from it.
  The  Private key is secret (the owner must keep it safe and never share it  with anyone). Instead, the Public key is public (the owner can share it  with whomever he wants).
  In  order to be able to decrypt each other's encrypted messages, Alice and  Bob must have each other's Public key: that is, Alice must have Bob's  Public key and Bob must have Alice's Public key.
  The following is a synthetic description of how PGP works (you don't need to understand how it works in order to use it):
  If  Alice sends a PGP encrypted message to Bob, PGP automatically creates  an 'envelope' for the message. This 'envelope' is locked by a  randomly-generated Session key. PGP symmetrically encrypts this Session key with Bob's Public key (which Alice has).
  When Bob receives the encrypted message,
  PGP uses his Private key to decrypt the Session key (which was encrypted by Alice with Bob's Public key);
  he then uses the Session key to decrypt the message (i.e. to open the 'envelope').

Practically,  to use PGP Private/Public key encrypted communications, it's enough to  have the Recipient's Public key and to follow the guides illustrated  below.