EVE PLEX can be traded between players on the in game market. It can also be used in the New Eden Store to upgrade your account to Omega Clone state, purchase virtual goods, and activate other account services.
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).