For the community's dictionary
CAS is the short version of Create A Sim and it refers to the special character creation menu in the games where you create your sims.
Every game so far has had its own version of Create a Sim. From the very simplistic style in The Sims 1 where you could just select between a few pre-made appearances to the much more advanced version in The Sims 4 where you can shape almost every aspect of your sim's body by simply clicking and pulling.
Different games offer different customization options, going from very simplistic and limited to basically limitless in later games thanks to more advanced game mechanics as well as custom content and mod creators.
NOTE! CAS may also be referring to Create A Style, a tool in The Sims 3 that makes it possible to completely alter and change the color and pattern of nearly everything in the game. From clothes and hair color to furniture, floors and wallpapers.
This might seem confusing, but thankfully, it's usually very easy to figure out which meaning it has through context.