the atp is the last certificate/license (depending on what country you take it in) you will get. an atp is required to be the pilot-in-command (captain) in a two-pilot environment. the requirements are either 1200 hours total or 1500 hours, also depending on where you take it. there are also some other requirements in flight time before you can go ahead and do the checkride.
some companies do require you to have a atp (or at least the written theory exams) before you will get hired as a copilot. this because they intend to train you towards becomming a future captain. i have been told that the atp is equivalent with a ph.d.
most people (like me) can't afford to buy the time up to this amount, so what they do is that they start off with a private (ppl), then go for commercial and an instrument rating, and finally getting a job to build the hours.
i fly by myself and have no requirement to have an atp although i will do the checkride when i get the chance. most pilots will never take that last certificate/license. the atp stage is education too!