Companion planting?

New concept for me...companion planting. I read a little bit about it tonight on-line. Does anyone out there know much about it? I am thinking it sounds like a way to maximize your soil usage and the amount and variety of food you grow, but it also seems like there are some plants that grow well together, and some that don't. Any thoughts or experience appreciated.

I also planted today (check out some of the seedlings that finally made it into my beds). During this process it occurred to me why it is better to plant some seeds directly into the beds (rather than sprouting in individual cups). You plant a continuous row of seeds (because they are very, very small seeds, like lettuce or carrots), and then you thin them out once they sprout. This does not work so well when you sprout the seeds in individual cups. Not sure what to do with the lettuce clumps I inadvertently produced?!