One region of the brain that contains a lot of THC receptors is the hippocampus, which processes memory. When THC attaches to receptors in the hippocampus, it weakens short-term memory.
The hippocampus also communicates with other brain regions that process new information into long-term memory. (That's how you can remember today's math lesson or a new friend's phone number.) In the brain, under the influence of marijuana, new information may never register - and may be lost from memory.
Maybe you've heard that in some people, marijuana can cause uncontrollable laughter one minute and paranoia the next. That's because THC also influences emotions, probably by acting on a region of the brain called the limbic system.
And don't forget this: THC can make something as simple as driving a car really dangerous.