The tag sums-of-squares has empty tag-wiki at the moment.
Based on the name, I would expect to see there topic such as Fermat's theorem on sums of two squares or Lagrange's four square theorem etc. (In general, mostly topics from number theory.)
But when you have a look at question actually using this tag, you will find there also questions about the sum $\sum_{k=1}^n k^2$, equations containing squares, various infinite sums.
What would be reasonable content for this tag?
I think that if this tag is supposed to be useful, then we should clarify what kind of questions it should be used for and created a reasonable tag-wiki describing this tag.
EDIT: (Jan 16 2014) I have created tag-excerpt and tag-wiki based on number-theoretical meaning of the term. Of course, if further discussion shows that we want use this tag also for some of the other meanings of the term sum of squares, we can include them in the tag-wiki and tag-excerpt, too.