Email (or e-mail) is a federated system of sending messages from one sender to one or more recipients. While the concept began in the 1960s, since 1982 "email" almost universally refers to messages transmitted with the SMTP protocol, and other electronic message/mail systems generally have outdated and/or proprietary names.
Composing and Sending
An email message can be composed in any application, with the payload of the message historically being plain text but in recent decades also often rich text and much more rarely including multimedia content through extensions to the protocol and other standards.
Once composed, an email client is used to add metadata to the message such as the recipient(s), the content type, importance, and other less common attributes. In a federated system a recipient is identified by a name and a server. Most commonly with email recipient names are short plain text strings ("bob", "administrator", "cat_lover_123") and servers are identified by an internet domain name (yahoo.com, physics.college.edu, etc). Less common variations on this mostly occur in local area network environments or when using proprietary software that adds additional features or translation layers.
With this metadata added, the email client will then hand the message off to an email server for handling and eventual delivery. See below for more details on that.
Email is one of the few communication systems old enough and well enough established to be enshrined in the most fundamental layers of internet service identification. Messages destined for college.edu do not necessarily go to the server that domain name points at. Instead, that domain can have a special record in the domain name system (a DNS MX record) that indicates another server handles its incoming mail. This information must be used at some point in the delivery of the mail.