The show generally has two angles. Firstly, Andy Millman (Gervais) is trying desperately to break through, which means a line here and there or any obvious camera presence. His luck is not improved by having a horribly inadequate agent (Merchant). Also, Maggie is a bit bumbling and more often than not puts Andy in rather precarious situations that result in him losing more than just face.
The other strand that runs throughout is their normal lives. Just as The Office wasn't purely about office antics, and touched on the lives of the people, so too does Extras delve into not only their personal lives, but also the relationship/friendship between Andy and Maggie.
The series also features different real life 'stars' and more often than not, these actors play integral roles in the shows story, as opposed to simple cameos.