What is a treatment?
A treatment is a narrative version of the story in a script, presented in story format, describing the main action with little or no dialogue. Many writers use treatments as a way of fleshing out their stories in narrative format to map out the flow of the action across the major scenes in the script before they start work on the dialogue. This way the writer can be sure that the story flows correctly before they commence the actually script. Most treatments are no more than 2-5 pages.
As tv writer, you're the person who'll spend the most time with your characters and their stories. If you're a movie writer, you'll be the one creating the world and then losing it - especially if, as usual, you're not the one asked to write the sequel, or even the shooting draft. If you're a tv writer, you can get to play in the world you and your fellow writers have created for as long as your show runs.
That's fun! It also means that tv is about twice as hard to write as movies. How do you find a format that can sustain 100 episodes? How do you create characters who will be consistently compelling over that many stories? How do you find a tone you want to sustain from fall through to spring? How do you lure the same viewers back week after week?
The answer is in the template. Every tv show, whether drama or comedy, fiction or 'reality', has hidden rules that define what it does - the format of storytelling, the goods that every episode must deliver. Delivering those goods according to template's hidden rules, while still keeping your stories surprising and compelling, makes tv writing hard! Delivering those goods regularly and on time, facing notes from everybody from the assistant director to the head of the network to the actor calling from the set, makes tv writing harder! If writing a movie is a one-night stand, writing tv is like a marriage! There are a lot more rules, and you have to work at it harder! But you get an experience that's richer and deeper, that can fulfill you for years or even decades.
To write tv, you have to know about twice as much as you need to know to write a movie. You need to know everything a feature film writer needs to know, plus a whole other basket of knowledge about "act outs", "core cast", character revelation, and how to play nice in writing room.