E-mail works as a "store-and-forward" application. Mail messages (and the identifying information - sender, receiver, time stamp) are stored on an e-mail server (post office box) until the recipient retrieves their mail. Typically, e-mail messages are short communications. Current e-mail systems also allow users to "attach" larger files of different types (documents, pictures, movies) to their messages. These attachments can also be retrieved or "downloaded" along with the e-mail message. In this way, e-mail services are similar to file transfer services on the network.

E-mail systems have evolved together with networking technology. The rapid growth of the Internet has allowed more and more people to connect "online". This allows for immediate communication between users of the network. The store-and-forward nature of e-mail systems does not require that the recipient be connected when the e-mail is sent to them. They can pick up their e-mail at a later time. In addition to e-mail, the Internet has spawned a variety of instant messaging systems that allow network users to "chat" without delay. This capability is referred to as "real-time", and is used when two or more users are connected to the network at the same time.