The phrase "a lot" means "a large number of the creatures, items, or objects in question". It is used in sentences such as, "What a lot of chocolate bars!"
The word "alot" isn't a word. It means nothing. Don't ever use it. Ever. I mean it. It's seriously annoying to people who speak, read, and write actual English. Actually, it can mean anything you want it to mean, if you accept that it isn't really a word. There's quite an interesting post about the creature known as the alot on the blog Hyperbole and a Half.
Told you this was going to be a short post. ;-)