The RNIB have put together a really useful guide to accessibility on Apple devices, aimed at blind and low-vision users. There’s lots of detail in there for Mac and iOS devices, with overviews of features like VoiceOver, zooming and type-ahead, as well as tips on things like how to alter contrast and invert screen colours.
You can download the guide by following this link. Only disappointing thing about this document is that it’s only available in RTF and Word formats. Would be nice if RNIB would put it online as an HTML guide.