EVE PLEX can be traded or sold between players on the in game market. It can also be used in the New Eden Store to upgrade your account to get Omega Clone state, purchase virtual goods, and activate other account services.
500 PLEX is valued at $0.0399 Cents each. This is a medium valued cost package and is intended as a mid way value if you wish to sell PLEX for ISK. 500 is enough to give you Omega status in game.
To use PLEX in the New Eden Store, It must be placed in your PLEX Vault. The Vault is a secure location in your inventory, shared across all characters on your account, where it can be placed for use in the New Eden Store. You can find the Vault in your inventory in the Neocom (Upper left corner of your game screen).