You mean how you can spot one on a compound? It's merely the part of the compound that branches away from the main chain.
Let's say you had this, N,N-dimethylacetoacetamide:
You could say that the main chain is the carbon attached directly to the nitrogen from the left (carbon-1) all the way to the far left carbon (carbon-4).
Then, the carbonyl groups would be the #"C"="O"# of carbon-1 and carbon-3. The only other functional group is the secondary amine group on carbon-1. You can tell it's secondary from my writing N,N, which implies two substituents on the nitrogen. You could also say that the carbonyl on carbon-1 along with the #"N"("CH"_3)_2# can be coupled together as one functional group called an amide.
If you want to be able to remember these, try quizzing yourself with flash cards.