MathJax is a JavaScript engine for displaying mathematics, available on some Stack Exchange sites.
MathJax is a JavaScript library that is used to display mathematical equations on the Stack Exchange sites where it is activated. You can ask questions about the use of MathJax on Stack Exchange here (though you might prefer Mathematics Meta, where you can use MathJax in your question and the author of the project participates).
Questions about MathJax itself are off-topic here — ask only questions about its use on Stack Exchange. If you're making a website and have trouble with MathJax, try Stack Overflow. If you have trouble with MathJax rendering in your web browser, try Super User.
MathJax is only activated on sites where it is common to need mathematical equations, including:
- Biology
- Chemistry
- Computational Science
- Computer Science
- Electrical Engineering
- Mathematics
- MathOverflow
- Physics
- Psychology & Neuroscience
- Theoretical Computer Science
See this question for the complete list.
Tutorials
Mathematics has a short tutorial, MathJax basic tutorial and quick reference.
Operations Research has a MathJax FAQ, FAQ: How do I use MathJax on this site, which markup extensions are supported?.