An account is simply a leaf node in the account tree. Each account is connected to exactly one address, so you may hear the words "account" and "address" used some what interchangeably, which can get quite confusing since Bitcoin has it's own special definition of account which is different from the type of account being described on this page. Each account holds 5 values: address, balance, age, limit, futurelimit. When the withdrawal limit of an account is updated the age will be updated, and the new limit value will be saved in the futurelimit field. When the age is more than 100 blocks futurelimit will be copied into the limit field the next time a transaction is made.