Plumbob

A PlumbBob is used in most titles in the Sims series to identify the character you selected, and is also used to show the mood of your Sim. In The Sims, the plumbob is bright green if the Sim has a high mood, bright red if it is low, or if mood is somewhere in between then it will be a shade of brown reflecting the approximate level.

Also in TS2, Sims who belong to the same active group as the selected sim will have a smaller plumbob over their head. This plumbob is blue, irrespective of mood.