Blend

From GPWiki

Blender uses its own internal fileformat that is constantly evolving. A .blend file is basically a dump of the datastructures used in blender with some headerinfo that is used to tell blender how to deal with that version of a .blend If your interested in what it actually looks like the best thing you can do is download the blender source and look at the following file: blender/source/blender/makesdna/intern/makesdna.c

You probably will also want to read the following document which expains a bit about how blender works with these datastructures and what they look like: http://www.blender.org/development/architecture/