Philosophically, though, it's not simple at all. If we assume that the world only has physical things in it and they are all subject to the same laws, with no Cartesian souls or other supernatural non-physical entities, then all events are caused by previous physical events. The brains and bodies of animals are merely collections of physical particles; therefore every physical state of an animal, including its brain-states (which may correspond to mental states) are the result of previous physical states of the brain and their interactions with the external environment (along with—perhaps—probability too). As long as we believe that the laws of physics govern everything that exists in the world (even if we don't have that complete description of those laws yet), then mental events never become part of the causal chain of one physical event leading to another, and the behavior of any being, no matter how complex the mental properties it may exhibit, can be described simply through physical laws.
Okay that's a bit jargon-y. Put another way: as long as there is such a thing as the laws of physics, and as long as every particle in our bodies obeys the laws, then those laws provide (or at least would seem to provide) a complete description of the evolution of events that takes place in our bodies—and brains. In this perspective, then, mental events are completely irrelevant with respect to causality.
But then we have a major conflict between the common-sense, cognitive view of mental life, and the strictly physical view, don't we? How can we resolve it?
Well, the first way would be to throw out the common-sense view and say that mental events really are irrelevant by-products, or "epiphenomena," of physical events, which a lot of people used to believe. On this view physical states of the brain give rise to our mental life, but that mental life exerts no causal influence back on the evolving physical states of the brain, and thus no causal influence on our behavior at all. But that would be crazy, because it would leave the mental completely outside the realm of biology and psychology. If mental events are causally irrelevant, then they couldn't be subject to natural selection, and it would be a complete coincidence that my thoughts and desires line up so well with my behavior. That coincidence is too improbable to take seriously.
The easiest solution is to say that the physical events in the brain and their corresponding mental events are identical. Thus to say that a physical event causes another physical event is exactly equivalent to saying that its corresponding mental state caused the next physical event, since the corresponding mental and physical event are one and the same. This theory is called, not surprisingly, "identity" theory of mind.
Although it's the simplest, most elegant, and I think, most correct explanation for mental causation within a physicalist framework, it's not the most popular theory among today's philosophers of mind. That award goes instead to "functionalism." The major difference between functionalism and identity theories of mind is that functionalism permits for "multiple realizability." According to functionalists a certain physical state can correspond to one and only one mental state, but a given mental state can correspond to an infinite number of possible physical states. The idea is that you don't need a human brain to get something to feel pain, but it could equally well happen in an animal with a completely different brain, an animal without a brain at all, or even synthetic robot, as long as the system is complex enough and the connections are all rigged up the right way.
Another time I'll explain the pro's and con's of identity theory of mind, functionalism, and why I think the latter doesn't work, especially when it comes to mental causation...woohoo I can feel your excitement....that is, if you're even having any thoughts right now at all!