what is materialism?

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It's the idea that everything in the world is tied to a representation of something physical. It means that minds, are simply an increased mental acuity, and are not separate from the body. This would mean that minds are part of the body, which excludes the idea of a metaphysical soul, excludes the ideas of minds without bodies (God, angels, etc.), and excludes anything that is not physical altogether.

There are two forms of materialism: methodological and philosophical.

Philosophical materialism is the belief that the universe consists of only material entities. Specifically, there are no supernatural entities such as God.
http://www.naturalism.org/tenetsof.htm "This version of naturalism, what might be called inclusive naturalism, asserts that the world is of a piece; that everything we are and do is included in the material continuum whose most basic elements are those described by physics. ... There exist no immaterial souls, spirits, mental essences, or disembodied selves which stand apart from the physical world. "

Methodological materialism comes from how we conduct experiments in science. Basically, we can't "control" for the possible existence of the supernatural. IOW, we can't point to a test tube and say "that one has no supernatural in it" and another test tube and say "that one has supernatural in it". Therefore, we are limited to looking at ONLY material causes and make no comment about whether the supernatural or other non-material entities (such as souls) exist.

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"If sound science appears to contradict the Bible, we may be sure that it is our interpretation of the Bible that is at fault." Christian Observer, 1832, pg. 437

"Christians should look on evolution simply as the method by which God works." Rev. James McCosh, theologian and President of Princeton, 1890