What's the difference between a pitch, an outline, a synopsis, and a treatment?
Pitches, outlines, synopses, and treatments are all useful for different purposes and in different ways.
As a freelance writer in film or in television, you will be expected to pitch. You come in to a meeting with a minimum of three story ideas; you don't want to have just one idea, because they might already have something like it in development. You aren't expected to have all the details of every story worked out, but you should have all the main points covered. In other words, know who the main character is, what he or she wants, and what stands in the way. Be able to tell the story with enthusiasm, complete with a beginning, a middle, and an end.
Here's the thing about pitching: we do it all the time. Every time you try to convince a friend or spouse to see a movie or TV show with you, given what you've seen in the trailer or read in TV Guide, you're pitching. You're taking a little bit of info and trying to convince someone else that a really great experience will follow. Get good at this, because you do it a lot.
An outline is something you do once you've been hired to write the film or the TV episode. It's a tool to help you write the film or the TV episode and (crucially) it's a step for which you get paid. When the time comes you'll need to do it, but you don't ever need to turn one in without a prior written agreement and a forthcoming check. If you don't use an outline when you write, there's plenty of time to learn how to do one after you've sold your pitch. On the other hand, I personally think an outline is invaluable as a writing tool; it gives you a place to work out all the structural problems before you start writing dialogue. But that kind of personal outline can be informal, messy, or even incomprehensible to outside eyes, because you're the only one who'll ever read it.
You will never need to write a synopsis, so don't sweat it.
Now, sometimes people ask you to send a synopsis of your movie for them to consider producing, but that's not really what they mean. What they want for you to send them is a written pitch, as opposed to a verbal pitch. Don't be fooled. A synopsis encapsulates a story; a pitch is designed to sell it. It's not about telling them this happens then this happens then this happens. It's about telling them this is so cool and then this is even cooler and then, Surprise!, this is the coolest of all. It's much less about the details than about the sizzle. So don't worry about synopses; instead, spend your time getting good at pitching both verbally and in writing.
Finally, the treatment. Here is the first rule of pitching: never, everleave anything behind. Ever. Some people will tell you there are legal reasons, and some people will tell you I'm all wet and you should have a treatment to leave behind. Ignore them. The reason you don't ever, ever leave anything behind is that when someone in the room likes your idea, what they'll remember are the bits they liked, the bits that worked, and their own spin on the project. They won't remember the bits they didn't like, that didn't work, and that they aren't invested in... unless you leave behind something in writing to remind them. Let them pitch the idea up to their boss with the enthusiasm that comes from their own investment in the story. In other words, from their own notes. If they like the idea, that's all you want. You've done your job. They'll do their job from there. Leaving something behind will only ever hurt your chances of selling the pitch.
Sometimes, when you're selling a movie pitch, they might request a treatment, and that's a little different. If they ask for a treatment before you've pitched, then again it's semantics: they don't mean a treatment, they mean a written pitch. But if you've pitched and they like the idea, and they tell you the next step is a treatment, what they're really looking for is a way to make sure you know how to execute the story. Reputable producers will pay you for this step; if they're not willing to at least give you an option agreement, I wouldn't write a treatment. You may work for free (free options abound, especially for first-time writers), but you should never write without a contract.
Finally, writing a treatment assumes that you have not already written the script. If you've already finished the script, that will be your calling card as a writer. There's no reason to send them a treatment of a finished script. Again, a "synopsis" (by which they really mean a written pitch) could be requested, but there's no reason to write a ten-page treatment for a finished script. That's just silly. If they're interested in the pitch, tell them it's a finished script and ask them if they'd like to read it. The answer will be yes or no. Remember, it's a numbers game: you may rack up a lot of No's, but you only really need one Yes.