Almost all real, modern math is typeset in LaTeX, which is what you're seeing (though it's not quite right--should be \infty). If you look at any of the papers on the arXiv, that's what they're written in. I understand physics uses it heavily as well, while many other disciplines manage to typeset their equations with MS Word (I shudder to think).
In fact, you're just seeing the "math mode" notation. LaTeX can be used to typeset a huge variety of things (letters, books, whatever). There are a bunch of ways to turn LaTeX into nicely typeset output. On the web, by far the most popular solution is to use MathJax. You can make your own HTML pages which render math mode LaTeX in seconds by adding a single script tag (I did that very thing a couple of days ago, in fact). I've linked a site where you can play around in a WYSIWYG editor without installing anything. There are also distributions for every major operating system. Among others, on Windows there's MiKTeX, on OS X there's MacTeX, on Ubuntu (say...) you can just install TeXLive, etc.
Most people find the learning curve for actual LaTeX very steep. For writing basic formulas, you don't need much though. _ means "subscript", ^ means "superscript", {...} are grouping symbols (e.g. try x^{a+b} vs. x^a+b in the editor I linked), \sum gives a big Sigma, \prod gives a big Pi, \int_a^b gives an integral with lower limit a, upper limit b, etc. The editor I linked has a graphical component that you can use to explore. Oh, $...$ means "inline math", though more properly it's \(...\) (nobody uses this). Similarly $$...$$ means "display mode math", like when a big equation is typeset on a line by itself, though more properly it's \[...\] (this is more common, oddly). These are sometimes omitted in online math.
People who are fluent in LaTeX do tend to forget others may not be. I've written literally hundreds of pages of LaTeX at this point, so the "indecipherable notation" you wrote is almost as easy for me to read as if it had been rendered. Most likely whoever typed it didn't even intend for it to be rendered (e.g. because of the infty vs. \infty issue) and just wanted to dash off something quickly and unambiguously. Sorry about that.