A decision problem is a problem whose solution is either yes or no given some input values. In other words, it is a problem that can be posed a yes/no question.
That there is a alogorithm that can solve the problem.
Class P is all decision problems that a sequential deterministic machine can solve in an amount of time that is polynomial in the size of the input. Class NP is all decision problems that can be solved in polynomial time on a non-deterministic machine.
Whether every problem whose solution can be quickly verified can also be solved quickly.
1 million $.