MathML (Mathematical Markup Language) is an industry standard adopted by the World Wide Web Consortium (W3C)  as the approved way of expressing math on the web. Like the more familiar HyperText Markup Language (HTML), which sets the standard for web browsers to display literary text, MathML is an international open standard for encoding math content. MathML is needed because HTML has no means of marking up mathematical expressions. Using MathML provides the highest level of accessibility to math within digital documents.
Before the advent of MathML, web authors often resorted to inserting graphical images, basically digital "pictures" of equations taken from other software products, into their documents. Such a practice is problematic because graphical images are inherently inaccessible. Although alternate text descriptions (commonly in the form of "alt tags") can be added to these images, this approach has drawbacks because text does not always provide true comparable access to information found in mathematical notation.
MathML is composed of a number of Extensible Markup Language (XML) tags that provide the means to mark up an equation in terms of both presentation and semantics, thus enabling the expression of the information content or meaning behind equations and not just the visual representation. MathML is therefore able to provide sufficient information and structure of the math to support both visual display and assistive technology access. MathML equations will, for instance, increase in size as users change font size to increase readability. MathML also allows the synthetic speech user to set different verbosity levels, automatically adjusts for the user's native language, and supports aural navigation through complex math equations for better understanding. For Braille users, MathML supports various Braille math formats, subject to Braille translation software support.
MathML is gaining widespread support among math editors, and converters have been developed for those that don't support MathML natively. Among the most popular web browsers, Mozilla Firefox directly supports MathML, whereas Microsoft Internet Explorer requires the plug-in Design Science's MathPlayer . MathPlayer displays MathML-based formulas visually within the browser and passes the structure and content of these formulas along to screen readers for audible rendering to students who are blind. Most major screen readers are compatible with MathPlayer.
Design Science, Inc. also produces a tool for creating accessible MathML equations, called MathType . MathType is a plug-in for Microsoft Word that enables authors to include MathML equations within their text documents. When exported as web pages, the pages include accessible math. Similar products include MacKichan Software's Scientific Word  and Scientific Notebook .
For more information on how to make math accessible consult the DO-IT Knowledge Base article How can I create math and science materials that are accessible to students with visual impairments? 
-  World Wide Web Consortium (W3C)
-  MathPlayer
-  MathType
-  Scientific Word
-  Scientific Notebook
-  W3C Math Home
-  W3C MathML Software List
-  How can I create math and science materials that are accessible to students with visual impairments?