Accessibility is all about ensuring that applications can be used by all people, and specifically that people with disabilities can perceive, understand, navigate, and interact with them. This is often mandated in law e.g. in the USA, Section 508 established binding guidelines for technology accessibility; as did the UK's DDA (Disability Discrimination Act), which is mostly now superseded by the Equality Act
When discussing accessibility, most of the time we focus on web accessibility (understandable given its global reach). For website accessibility, the W3's WCAG 2.0 guidelines are the first port of call and are referenced by most accessibility guidance.
A good question was raised yesterday on a project about the applicability of accessibility guidelines to desktop applications, specifically relating to software being supplied by a 3rd party and the level to which they can (or have to) prove it meets the law. The intent of the legislation is agnostic of the delivery method, and the WCAG guidelines can easily be used for checking non-web accessibility, which is a good start. Having 'web' in the name is unfortunate however as it's easy to point at that and say "this isn't a website".
Also it does seem that the focus in general is on the web because of the large potential reach, rather than desktop which typically will concern a smaller group of people, often internal to an organisation, (although who still need to be able to use the software they are given!) For desktop application accessibility, I haven't found any UK guidelines that refer directly to desktop rather than web.
In the US however the legislation is split into separate sections for web and desktop. The desktop guidance here looks very applicable to all sorts of desktop applications and a good guideline for any desktop software suppliers to be referring to. The benefits from following this are not just compliance with legislation and meeting the needs of a small part of the userbase, but can have a usability benefit to all users of an application
There's also a useful round-up of accessibility tools and guidance on the Paciello Group website
When developing desktop applications, it is recommended to refer to the user interface guidelines for that platform as well e.g.