Most people are familiar with the operation of ordinary mail. I write a letter, address it and deposit it with the postal service for delivery. Along with many other items it is sorted and transported towards its destination until it is deposited in a mail box corresponding to the address on the package. Ultimately it is collected from the mail box and read.
Electronic mail works in a similar fashion.
I write a letter on my computer, address it appropriately and send it to another computer to which my computer connects. The message is passed from computer to computer until it is deposited on the computer corresponding to the address. This system of message delivery by means of 'store and forward' can work on a local network or across the global Internet. It is one of the most basic services on the Internet and can be extremely useful as a means of communication. Advantages include speed of delivery which can be almost instantaneous and the capacity to work without both parties being available simultaneously.