If you've somehow made it here without knowing what a plural system is, we recommend these resources:

• morethanone.info is a clear and concise "plurality 101".
• The Plurality Playbook is a resource meant to "help employees and managers understand dissociative disorders, as well as plurality as a whole". It's workplace-oriented but can also function as a general guide.
• The Layman's Guide to Multiplicity also contains more in-depth explanations. Even though it's an older resource (last updated 2004) and the terminology is sometimes outdated and we don't agree with everything in there, we really like its debunks of common myths about plurality.