Suppose, for example, that we know that n pigeons have been housed in m pigeonholes. How big must n be before we can be sure that at least one pigeonhole houses at least two pigeons? The answer is the pigeonhole principle: if n > m, then at least one pigeonhole will have at least two pigeons in it. Ramsey's theorem generalizes this principle as explained below.
A typical result in Ramsey theory starts with some mathematical structure, which is then cut into pieces. How big must the original structure be in order to ensure that at least one of the pieces has some interesting property?
For example, consider a complete graph of order n, that is, there are n vertices (dots) and each vertex is connected to every other vertex by an edge (a line). A complete graph of order 3 is called a triangle. Now color every edge red or blue. How large must n be in order to ensure that there is either a blue triangle or a red triangle? It turns out that the answer is 6. See the article on Ramsey's theorem for a rigorous proof.
Another way to express this result is as follows: at any party with at least six people, there are either three people who are all mutual acquaintances (each one knows the other two) or mutual strangers (each one does not know either of the other two).
This also is a special case of Ramsey's theorem, which says that for any given integer c, any given integers n1,...,nc, there is a number, R(n1,...,nc;c), such that if the edges of a complete graph of order R(n1,...,nc;c) are colored with c different colors, then for some i between 1 and c, it must contain a complete subgraph of order ni whose edges are all color i. The special case above has c = 2 and n1 = n2 = 3.
Two other key theorems of Ramsey theory are: