If you see a LaTeX code above instead of a rendered formula, then your browser is probably rather old and should be updated or replaced (sorry!). The same formula rendered by wp-latex:
Both MathJax and wp-latex allow to type in LaTeX formulas directly in the WordPress blog editor. The MathJax rendering is scalable because Ajax makes it dynamic and adaptive. It looks nicer in modern web browsers. The wp-latex rendering is unscalable since it relies on a static bitmap image produced by LaTeX and dvipng. There is a similar story for images: SVG versus GIF|PNG|JPEG.
The installation of MathJax on WordPress is fairly simple. Take a look at the MathJax website!