What is Goldbach's conjecture?

Goldbach's Conjecture is that every even number greater than two can be expressed as the sum of two prime numbers. For example, 4 = 2 + 2, 6 = 3 + 3, 8 = 5 + 3, 10 = 7 + 3, 12 = 7 + 5, etc.

Although the conjecture has been checked up to very large values and many weaker results have been proved, the conjecture remains open. Because it is so well-known and easily understood, it is frequently the subject of mistaken "proofs" by amateur mathematicians.