What are decals for in Starcraft 2?
Solution 1:
Solution 2:
Take a close look at your buildings, these are little graphics on them, and possibly on some of your units too.
Solution 3:
According to Starcraft.wikia.com:
Decals are rewards, similar to achievements, used to signify the number of StarCraft II games a player has won. They appear on and around buildings and on at least some units.
Check the link for more specifics