SIDs are created when the account is first created in Windows and no two SIDs on a computer are ever the same. Users (you and me) refer to accounts by the account's name, like "Tim" or "Dad", but Windows uses the SID when dealing with accounts internally.
Here's an example of a user SID: S-1-5-21-1180699209-877415012-3182924384-1004. That SID is the one for my account on my home computer - yours will be different. It'll start with S-1-5-21 but will otherwise be unique.
See How To Find a User's Security Identifier (SID) in Windows for instructions on matching users with their SIDs.
A few SIDs can be decoded without the tutorial linked above. For example, the SID for the Administrator account in Windows always ends in 500. The SID for the Guest account always ends in 501.
You'll also find certain SIDs on every installation of Windows which correspond to certain built-in accounts. For example, the S-1-5-18 SID can be found in any copy of Windows you come across and corresponds to the LocalSystem account, the system account that's loaded by Windows before a user logs on.
While most discussions about SIDs occurs in the context of advanced security, most mentions here on my site revolve around the Windows Registry and how user configuration data is stored in certain registry keys that are named the same as a user's SID. So in that respect, the above summary is probably all you need to know about SIDs.
However, if you are more than casually interested in security identifiers, Wikipedia has an extensive discussion of SIDs here and Microsoft has a full explanation here. Both talk about what the various sections of the SID actually mean and list well known security identifiers like the S-1-5-18 SID I mentioned above.