NP Complete

Without getting into the mathematical details and jargon, we'll stick to informally defining the complexity class NP-complete. Problems in the NP-complete class are those problems in NP that are as hard as any other problems in NP. This may sound confusing but, before we get a chance to define NP-hard problems, one can safely assume that NP-complete problems are the hardest problems to solve in the complexity class NP.

By definition of NP, solutions for NP-complete problems can be verified in polynomial time. The wonderful property about problems in NP-complete class is that if we find a polynomial solution for any one of them, we will have found a solution for all of the NP-complete problems - thus implying P=NP. Take a minute to think through what we just stated. It is a very powerful assertion, claiming that the solution for one problem can unlock solutions to all other problems in NP-complete!

Conversely, if we can prove that no polynomial solution exists for any one of the problems in NP-complete complexity class, then we can say that no polynomial time solution exists for all of the problems in NP-complete - thus implying P≠NP.

NP-complete problems exhibit two properties:

They are in NP, i.e. given a solution one can quickly (in polynomial time) verify that the solution is correct.
Every problem in NP can be converted or reduced to a problem in NP-complete. Note that since all NP-complete problems are also in NP, by definition NP-complete problems can be reduced to each other also.

The second property allows us to say that if we can solve a problem A quickly, and another problem B can be transformed into problem A, then we can also solve B quickly. The caveat here is that the transformation itself should take polynomial time. If the transformation takes superpolynomial time then it defeats the purpose of transforming in the first place.

Both the graph coloring and subset sum problems are NP-complete, but string-matching isn't. Other notable NP-complete problems include:

Clique problem: A clique consists of a subset of those vertices in a graph, where each vertex is connected to every other vertex within the set. The goal is to find the maximum clique in a graph.
Hamiltonian path problem: Given an undirected graph is there a path in the graph that visits each vertex exactly once?
Sudoku: Solving a sudoku puzzle isn't polynomial time activity, whereas if given a solution to a Sudoku puzzle, the correctness can be verified in polynomial time.

NP-Complete and NP-Hard

NP Complete