I am looking at some code in C# and I don't know what it's doing. Can anyone show me the light?

ParentCategoryID == 0 ? "NULL" : ParentCategoryID.ToString()

I assume what's doing is returning "null" if ParentCategoryID equals 0 otherwise converting the ParentCategoryID to a string.

However, I don't understand how this construct works. specifically what is the ? function or the : function.

Thank you,
Paul