Materialism is the belief that all observable phenomena can be explained by material interactions.
Philosophers may disagree, but it is essentially the same as atheism, except that with materialism there is a possibility that deities do exist, it's just that they have no observable effect on the universe. (Note that from a scientific point of view, if something is not observable and has no observable effects, then that is the same thing as non-existence).
Some atheists prefer to describe themselves as materialists because (on face value) it is more a statement on what they do believe, rather than what they don't believe. However there is the problem that the word materialism has acquired the popular meaning: wanting to get many material possessions. Having to explain that you are referring to the former definition rather than the latter can be tiresome.