If you're a pure science or computer science major and googled a problem from your homework (who doesn't), then you've probably found the first four to five results linking you to Stack Overflow. If you've been impressed by its sheer number of resources then know that Stack Overflow is just one of the many websites that makes the popular specialized Q/A forum, Stack Exchange. 

What it is/has:
Where Stack Exchange differs from other Q/A sites is in its division of categories into sub sites or communities. SE has more than a hundred sites up and running and new ones are being opened every month or so. These sites specialize in topics like technology and science to arts, culture, business and much more. SE also emphasises on building communities rather than just running a forum. 

Stack Exchange, at its core, is technology oriented. Its biggest and most successful community is Stack Overflow that specializes in programming problems for both enthusiasts and professionals. In actuality, SE was originally Stack Overflow and was founded in 2008, and was later expanded in 2009. Stack Overflow is now the one stop destination for any programmer. The community is so established and used in the last eight years that new questions are often redundant since almost everything has already been questioned and answered. Aside from Stack Overflow, there are other communities that specialize in computer programming like Super Users (for power users), Server Fault (for system administrators), Programmers (for the conceptual side of programming), Cryptology, Ask Ubuntu, Android Enthusiasts and many more.

Stack Exchange also covers pure sciences like mathematics, physics, chemistry and even philosophy. There are dedicated communities for all the major religions and languages. There are also mediums for pop culture, science fiction, videogame and anime enthusiasts. There's Seasoned Advice (for amateur cooks), Photography, Travel, Graphics Design, Lifehacks, Writers, Home Improvement and even Parenting. 

Browse through SE's communities at: http://www.stackexchange.com/sites

One cool addition is their Stack Overflow Careers. This community consists of job seeking programmers and employers. Jobs can vary in durations, posts, salaries and companies. Employing companies have to sign up with a monthly fee to feature their organization. They can seek out aspiring programmers according to their merits or vice versa. Unfortunately it's not yet popularised in Bangladesh.  

How it Works:
You can ask questions as a guest (anonymously) or by signing up. SE's text box is quite enhanced. Alongside basic functions like bolding, italicizing, aligning and listing you can add external links, images, block quotes and sample codes. You can also add tags after the text.

Questions are upvoted if they're good and useful or downvoted if too silly and/or repeated. So it's better to search if your question was previously answered before submitting it. Answers have the same editor and voting system as questions. The questionnaire can green check one answer which has satisfied his/her question. There's also the scope for commenting on both questions and answers.    
SE has a reputation award process that you can access as a member. Reputation grants you many perks and privileges along the way, like answering, commenting and even editing someone's question. Reputation and awarding badges are a great way for encouraging members to contribute and building the community.

All in all it's a great community (or a bunch of communities) with plentiful resources and responsive people to get help from and also to join and help others out.