A system is a person with dissociative identity disorder (DID) or the first type of otherwise specifed dissociative disorder (OSDD-1), trauma at a young age caused their personality to not actually develop into one, building amnesia barriers and separate identities, or alters, that can form their own personalities, identities, opinions, and can even have different genders and sexualities. These identities are to protect the system as a whole, and can consist of trauma and emotion holders, protectors of many kinds, gatekeepers who hide trauma from the rest of the system and basically keep everything running, and persecutors who may reenact trauma or cause harm to the system (do NOT demonize the alters, they can be healed and become great protectors). There are some smaller roles like hygienest and caretaker but we personally don't have many of those? So other roles are things that you can study on your own time
There's also introjects (fictives, factives, and the lesser known songtive, fuzztive, and mixtive) which are alters that come from a source! Fictives are based off fictional characters, Factives are based on real people, songtives are based on songs, fuzztives are introjects who see themselves as none of these or in between, mixtives are mixes of sources. Then there's brainmades, which are just alters made by the brain. Some systems have more of one type of one type of alters, and that's a-okay!! For example, we have a lot of fictives because we also have ADHD and autism so we split from our hyperfixations and special interests (this is why puyo puyo is TAKING OVER THE SYSTEM, HELP ME.)
Another important term is the Host, which is just the alter who fronts the most. This may change, and alters may mask as this host, even going by their name full time.
(I think I know who confessed this as i have discussed something similar with a friend. if it is you hi mags)
Tl;dr - A system is multiple identities, called alters, sharing one body because of trauma
Feel free to ask any questions!!
-Ecolo (@ecoloo is my personal "alter blog" if you want to contact me specifically)