Rick's got a great post on what big and little endian are, and what the Apple switch has to do with Word for the Mac.
In the comments, Alicia asked about Windows...
I tried to make this a comment on his blog but the server wouldn't take it.
The answer is pretty simple: For Windows, there are parts of Windows that are endian-neutral (for instance, the CIFS protocol handlers and all of the DCE RPC protocol, etc), but the vast majority of Windows is little-endian.
A decision was made VERY long ago that Windows would not be ported to a big-endian processor. And as far as I can see, that's going to continue. Since almost all the new processors coming out are either little-endian, or swing both ways (this is true of all the RISC machines Windows has supported, for example), this isn't really a big deal.