The account class represents a player's server account. You can get the account object associated to any client using getPlayerAccount.
Accounts are unique to each client and can be used to store information that is persistent across map changes and user sessions. Clients that join without an account are given a temporary 'guest' account. This can store information like any other account, but isn't saved across sessions.
When a user logs in or out, the account object assigned to them will change. As such, you must not assume that the account attached to a client remains constant during their session.
PHP code to check password hashes from the MTA server database is here.