Treatments and outlines are a written way of tricking a production company into spending a large amount of money on an idea you've had that might turn out to be complete rubbish.
Theoretically, the idea goes like this: during a meeting your agent has managed to get you with a hapless representative of said production company, you throw together enough random phrases and current new sound-bites that eventually, by sheer chance, you come up with something sounding vaguely credible. Or ''It's about a dead radioactive Russian dissident who shares a flat with... Denise Van Outen!"
The representative likes your Van Outen idea (horribly likely actually), so you go away and write a outline - usually a couple of pages, but it could be four or five - and rewrite it until the production company are confident enough to give you some money for it. The fools! Possibly this is a few hundred quid that gives them the rights to shop it about the channels for six months, or they might commission you to write a more detailed treatment (could be eight to twelve pages) which will contain details of the main characters, the setting, and probably rough plotlines for all prospective episodes.
If they like this, you then get commissioned to write the first script. If they like that you're in pilot territory, and that's a whole different world.
So theoretically it goes: outline, treatment, pilot script.
The problem with this system is that if a script gets generated this way, it's essentially been assembled by committee even if the writer's name is the only one on it. In order to try and get through each stage, you're tempted to file off the rough corners, take out anything that sounds a bit controversial (or if it's a certain kind of comedy, put stuff in just because it is controversial). So ideally, the treatment you're writing is for a series that you know the production company wants to make, but leaves you with enough space to write the series you want to make, which is probably slightly different.
You'll notice I'm writing all this from the point of view that all writers are all creative geniuses, and all producer-type people are stunted money-grabbing know-nothings, whose only aim is to thwart your artistic vision. This is, of course, mostly false, but it does contain just enough of a grain of truth to help you sleep at night. Producers, of course, aren't allowed to point out publicly that most of the writers they work with simply aren't that good. This stops most of them sleeping nights at all, because of the rage and frustration, but at least they're paid more money than writers.
And if they're not, why are you hanging around with them?