A system is often a umbrella term for someone who has DiD, OSDD, UDD, P-DID, Polyfragmented DID, etc. While DiD (Dissociative Identity Disorder) and OSDD (Otherwise Specified Dissociative Disorder) are the two more commonly talked about/researched ones, there are many other kinds of a Plural Dissociative disorders. A system could only have two alters, or have thousands, there is no upper limit. https://www.google.com/search?q=what+is+DID&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari

An alter is a Part or Being in the system, The easiest way to explain to a singlet (non-system) is they are a "different personality" which is incorrect and outdated. but it is a start on where to learn. An Alter can be anything, they could be a factive of your childhood friend, a fictive of Finn from Adventure time, a house plant, a dog, anything. While it is common for alters to have a job or role, such as a emotional protector, a Gatekeeper, or a caretaker. some may not have direct jobs, some may be just memory holders or just around to hold fragments. some alters stay in the headspace and never front, and some alters can be forever in front and/or front frequently.

A headspace is a place or area in the brain so to speak, a really good visual example is the movie "Inside Out". A headspace can be anything, but more frequently it can look like a childhood home, a cabin, a hotel, a endless hallway of doors. somewhere of comfort that alters can stay. While some headspaces can be vast places like cities, towns, islands, continents, planets, etc. Not all systems have headspaces though, we don't know exactly how to describe that situation as we have a headspace, but not having a headspace does NOT mean a system is faking.

A Fictive is an Introject alter from a fictional source, perhaps a fictive of Pomni from TADC, or Jabberjaw from Jabberjaw (1976). A Factive is and Introject of a real person. Perhaps Markiplier, or maybe Drake, or a childhood best friend. It doesn't matter how new or old a Fictive/Factive's source is. its still valid. the media could only be out for 2 weeks ( or less) and that's still valid if that's an introjects source. It also doesn't matter if the source is "problematic" or not, it is not their fault for existing. From Bluey, TADC, Welcome home, to DSMP, Homestuck, Hamilton, QSMP. it DOES NOT matter what their source is. they are valid for existing. Source shaming isn't cool or funny. its rude and solves nothing.