A decision problem is a subset $~$D$~$ of a set of instances $~$A$~$, where generally $~$A$~$ is the set of finite bitstrings $~$\\{0,1\\}^\*$~$\.
Why is it called a decision problem? As a reader looking for an intuitive understanding, that's one of the first questions I want answered.
Is it about deciding things? Is "Where should we go to eat tonight?" a decision problem? :-)