LSD is basically a serotonin agonist. It's chemical structure is very similar to serotonin (5-hydroxytryptamine) so it fits into receptors in the brain that were meant to interact with serotonin (5ht). One of the things 5HT does is help control sensory input and route it to the different areas of the brain. LSD isn't exactly like serotonin chemically so those signals to the different areas of the brain aren't normal. Thus a visual stimulus such as a color might be sent to the wrong area of the brain and might be experienced as a sound (Hearing a color, etc, synesthesia, blending of the senses). And because LSD is a serotonin agonist, many of the effects of serotonin are blocked. This is perhaps why people's moods can flucuate so easily when they trip (happy then sad then exicted, 5HT is the main mood stabilizer in the brain). This might also explain the wierd effects like perspiration and sweating (serotonin paritally regulates body temp).
This doesn't fully explain the effects of LSD or serotonin and I can't promise any of the above is true...