The main theme of the movie is around the "Dream Sharing" theory. This theory is originated in the Project Somnacin, which doing research regarding to (lucid) dreaming and dream sharing. Somnacin, which is the drug used in this project, have the effect of initiate dream sharing (of people who have this injected).
In the movie, The Portable Automated Somnacin IntraVenous Device (PASIV) is designed to have a automatic and safe way to deliver Somnacin. (The user manual of PASIV is in here)
Now let's look at the team, Leonardo DiCaprio (as Dom Cobb) is leading a team in the dream world, which consists of:
The Extractor
The Point-man
The Forger
The Architect
The Tourist
The Mark
The Chemist
Like special forces in the real world, everyone has his/her specialism and duty, contribute and achieve the same goal.
According to the conversations between extractor and chemist, sedative Somnacin (to enhance dream stability) make the normal way (of waking up) doesn't work anymore, which is died in dreams. Instead, a kick must be implemented in order to wake sleepers up in each level of dreams. Hence, one of the team members have to stay at certain level while others keep going down, and this is a risky plan actually.
Finally, if you have watched Inception and still not know what's going on, you can refer to here.
