A language-independent collaboratively edited question and answer site for programmers.better explained as q/a site spun by the community of active developers.Anyone can ask questions regarding any programming language and there are many devs who will collaboratively work on the question to provide answers.
There are many programmers forums available and stackoverflow is unique among them as it is backed by active community of developers. Stackoverflow recently launched their careers site called Careers 2.0 which will help recruiters to hire the developers who are talented enough to answer the questions.
Not everyone who has an account in stackoverflow will get a job , you need to get higher reputation to compete for a job.For getting more reputation you need to answer well and thats how the community works on.As you gain reputation points you will earn badges as in foursquare.
The success of the community has made Google to announce the android tag as the official developer support forum. Stackoverflow.com is developed using ASP.Net 3.5 using MVC framework.Now the website gets 70 Million visitors per month and have 51 websites in the stack exchange network.