A system is a group of selves that share one body. This is also known as plurality or the state of multiplicity. These "selves" are sometimes called alters, headmates, or parts, depending on that system's personal preferences. For example, we prefer the term "headmate." Some plural folks may have only two headmates, while others may have an infinite amount. Just like many other things regarding systemhood, it's different for every system!
Headmates in a plural system often take turns having control of the body (this is known as "fronting") and cooperate with one another to accomplish goals. In our system, we talk to each other a lot, and have complex relationships with one another, just as we would with "real-world" friends. We're a team!
If you'd like to learn more, try checking out these resources.