Email (short for electronic mail) is a communication network application which involves sending a message from an originator to a 'mailbox' on the destination host system. The recipient runs an application program to retrieve it and view it at a time of their choosing. It was the first 'killer app' on the ARPANET, the first packet switching data network (although email existed between users of a single time-sharing mainframe before then).
There are two protocols involved in modern email.
One covers the format of the message (which is divided into 'headers' and 'body'), which has remained generally the same since the ARPANET days (albeit with extensions for security, multi-media content, etc).
The other, Simple Mail Transfer Protocol, covers the method used to carry the message from the source host to the destination (possibly via one or more 'relay agents'). Although this also dates back to the ARPANET (although it has been updated to function over TCP), it was a replacement for an earlier protocol used for email carriage, which was an extension to FTP.