Screenwriting for television is one of the most lucrative careers in show business. It is not so different from writing for feature films, but with a few differences. Unlike film, for example, television shows run for years instead of two hours. As such, there are more rungs on the ladder for a television screenwriter.
To start at the first step, most TV screenwriters usually take jobs as writing assistants. Writing assistants get to shoulder minor responsibilities in the overall screenplay, but what's important is learning how the business works and what goes on in a set. The salary would be minimal, but at this stage it's not the money that counts as much as the experience.
Freelance screenwriters usually submit spec scripts to network executives or producers just like in writing for films, except that TV writers usually write their spec scripts based on an existing television show. Why is this, exactly? As said before, television shows run for years. A writer would have to come up with new, interesting episodes week after week, and remain consistent in writing the characters. It is almost impossible to have just one writer in one show and, as such, most TV series have a staff of numerous contributing screenwriters. When a freelance screenwriter submits a spec script for an existing television show that captures the series well and has the potential to be realized on screen, the network hires him to be a staff writer.
The highest position for a television screenwriter is to be a television series showrunner. This means that the writer is the one who creates the actual premise and idea for the show. The showrunner heads his writing staff and runs through and approves all creative decisions not at all a bad place to be in one's career. Of course, a showrunner would have to pitch his ideas to particular network executives first. Even after the show has been approved and is being aired on TV, unlike the film industry a writer can never be sure how long a series will last. Ratings are vital to the first few seasons of a show, and writers should be able to come up with a series that would hold people's attention enough to generate fans.
Nearly all popular television shows today follow this format. House, for example, is owned by the Fox Broadcasting network. Its series creator and showrunner is David Shore, but not every episode is written by him. On a medical serial like House, the writing staff is full of initial writers, rewriters and researchers.
It is true that television writing is fickle though once a writer breaks into the industry, it does get easier as he goes along. It's a job that is hard to maintain as they come, but fulfilling in its long-term benefits if one's stories can reach millions of viewers a day, what else can be expected?