The Makernote is tag 0x927c in the Exif IFD,
Exif.Photo.MakerNote. According to
the Exif 2.2 standard, the makernote
is "a tag for manufacturers of Exif writers to record any desired information. The contents are
up to the manufacturer, but this tag should not be used for any other than its intended purpose."
Most vendors do not publish their makernote specifications. Most of the specifications available on the Internet were reverse-engineered. Judging from the size of many makernote fields, manufacturers store a lot of information there and from the available specifications, some of this is certainly quite interesting. For example, makernotes may contain information about the lens used, contrast, saturation and sharpness settings, image quality settings, etc. For some more advanced camera settings, the Exif standard doesn't define a standard tag, so camera vendors have to resort to using the makernote. Unfortunately, in some cases, camera vendors store important information only in proprietary makernote fields, instead of using available Exif standard tags. The most infamous example for this is Nikon's ISO settings tag.
Most vendors write the makernote in TIFF format, i.e., in the same format as the rest of the Exif information is encoded. This appears to be a sensible thing at first glance. Unfortunately, in general it means that any change of an Exif tag, which moves the makernote field, will corrupt it. It is an inherent problem of the TIFF format that a writer must know the format and all extensions used, in order to be able to write changes correctly; unknown tags are potentially corrupted when they are moved (rearranged). But since makernotes are usually proprietary, Exif writers often don't know these details. The reason to write to the Exif data could be as simple as to add copyright information, an Exif comment, etc. Some camera manufacturers seem to have recognized this problem and now use a modified TIFF format with offsets relative to somewhere at the beginning of the makernote field for the makernote IFD to address the issue.
The following table summarizes the structures of the makernote field used by some vendors. Please let me know if you have additional information.__makernotes__
Exif.org has another table with similar info and sample pictures: Digital Camera Sample Images. According to this source, (at least some) Ricoh and Kodak cameras do not write the makernote in IFD format.
file format by TsuruZoh Tachibanaya
 EXIF Makernote of Canon by David Burren
 Makernote EXIF Tag of the Nikon 990 by Max Lyons
 "Makernote" Exif tag of Casio by Eckhard Henkel
 Minolta MakerNote by Dalibor Jelinek
 Sanyo MakerNote by John Hawkins
 SIGMA and FOVEON EXIF MakerNote Documentation by Foveon
 Panasonic MakerNote Information by Tom Hughes
 Various Makernote specifications from the PHP JPEG Metadata Toolkit by Evan Hunter
 ExifTool, the most complete and up-to-date tool for reading and writing meta information in image files, by Phil Harvey