Header
0 - 1   2 "BM"
2 - 5   4 File size
6 - 9   4 Padding
A - D   4 File offset to Raster Data (H36)

Info Header
E - 11  4 Size of Info Header (norm=D40)
12 - 15 4 Bitmap Width
16 - 19 4 Bitmap Height
1A - 1B 2 Number of planes (def=1)
1C - 1D 2 Number of bits per pixel
1E - 21 4 Compression (0=none) (1=8bit) (2=4bit)
22 - 25 4 (compressed) Size of Image/Raster Data (may be set to 0 if compression 0)
26 - 29 4 Horizonal Resolution (pixels/meter)
2A - 2D 4 Vertical Resolution (pixels/meter)
2E - 31 4 Number of important colors (0=all)
32 - 35 4 Colors important

Color Table (present only in 8bit, 4bit, 1bit)
1 Red Value
1 Green Value
1 Blue Value
1 Padding

Raster Data (pixel data)
Pixels are stored bottom-up, left-to-right.
Pixel lines are padded with zeros to end on a 32bit (4byte) boundary.
For uncompressed formats every line will have the same number of bytes.
Color indices are zero based, meaning a pixel color of 0 represents
the first color table entry, a pixel color of 255 (if there are that
many) represents the 256th entry. For images with more than 256 colors
there is no color table. 

Raster Data encoding for 1bit / black & white images
BitCount = 1 Compression = 0  
Every byte holds 8 pixels, its highest order bit representing the
leftmost pixel of those. There are 2 color table entries. Some readers
will ignore them though, and assume that 0 is black and 1 is white. If
you are storing black and white pictures you should stick to this,
with any other 2 colors this is not an issue. Remember padding with
zeros up to a 32bit boundary (This can be up to 31 zeros/pixels!)  

Raster Data encoding for 4bit / 16 color images
BitCount = 4 Compression = 0  
Every byte holds 2 pixels, its high order 4 bits representing the left
of those. There are 16 color table entries. These colors do not have to
be the 16 MS-Windows standard colors. Padding each line with zeros up
to a 32bit boundary will result in up to 28 zeros = 7 'wasted pixels'.

Raster Data encoding for 8bit / 256 color images
BitCount = 8 Compression = 0  
Every byte holds 1 pixel. There are 256 color table entries. Padding
each line with zeros up to a 32bit boundary will result in up to 3
bytes of zeros = 3 'wasted pixels'.

Raster Data encoding for 16bit / hicolor images
BitCount = 16 Compression = 0  
Every 2bytes / 16bit holds 1 pixel.   
<information missing: the 16 bit was introduced together with Video
For Windows? Is it a memory-only-format?>  
The pixels are no color table pointers. There are no color table
entries. Padding each line with zeros up to a 16bit boundary will
result in up to 2 zero bytes.

Raster Data encoding for 24bit / truecolor images
BitCount = 24 Compression = 0  
[Edited by The Mega-ZZTer, because this part was WRONG]
For each pixel, the color is written in BGR order... yeah, in reverse
(I hate Microsoft, it's all their fault :P).  There is no padding until
the end of a row, then it appears to pad to the nearest 4 bytes... I
dunno for sure tho, but sometimes there is a padding of 0, 1, 2, or 3
that I've seen, so I assume it's to the nearest four... I suggest you
experiment.

Raster Data compression for 4bit / 16 color images
BitCount = 4 Compression = 2  
The pixel data is stored in 2bytes / 16bit chunks.  The first of these
specifies the number of consecutive pixels with the same pair of color
The second byte defines two color indices. The resulting pixel pattern
will be interleaved high-order 4bits and low order 4 bits (ABABA...).
If the first byte is zero, the second defines an escape code. The
End-of-Bitmap is zero padded to end on a 32bit boundary. Due to the
16bit-ness of this structure this will always be either two zero bytes
or none. 

n        c          Description
(byte 1) (Byte 2) 
  >0       any      n pixels are to be drawn. The 1st, 3rd, 5th, ...
                    pixels' color is in c's high-order 4 bits, the
                    even pixels' color is in c's low-order 4 bits.
                    If both color indices are the same, it results
                    in just n pixels of color c 
   0       0        End-of-line 
   0       1        End-of-Bitmap 
   0       2        Delta. The following 2 bytes define an unsigned
                    offset in x and y direction (y being up) The
                    skipped pixels should get a color zero. 
   0       >=3      The following c bytes will be read as single pixel
                    colors just as in uncompressed files. up to 12
                    bits of zeros follow, to put the file/memory
                    pointer on a 16bit boundary again. 

Example for 4bit RLE Compressed Data Expanded data 
03 04               0 4 0 
05 06               0 6 0 6 0 
00 06 45 56 67 00   4 5 5 6 6 7 
04 78               7 8 7 8 
000 02 05 01        Move 5 right and 1 up. (Windows docs say down,
                    which is wrong) 
00 00               End-of-line 
09 1E               1 E 1 E 1 E 1 E 1 
00 01               EndofBitmap 
00 00               Zero padding for 32bit boundary 

Raster Data compression for 8bit / 256 color images
BitCount = 8 Compression = 1  
The pixel data is stored in 2bytes / 16bit chunks.  The first of these
specifies the number of consecutive pixels with the same color. The
second byte defines their color index. If the first byte is zero, the
second defines an escape code. The End-of-Bitmap is zero padded to end
on a 32bit boundary. Due to the 16bit-ness of this structure this will
always be either two zero bytes or none.   
 
n        c        Description 
(Byte 1) (Byte 2)
  >0       any    n pixels of color number c 
  0        0      End-of-line 
  0        1      EndOfBitmap 
  0        2      Delta. The following 2 bytes define an unsigned
                  offset in x and y direction (y being up) The skipped4
                  pixels should get a color zero. 
  0        >=3    The following c bytes will be read as single pixel
                  colors just as in uncompressed files. A zero follows,
                  if c is odd, putting the file/memory pointer on a
                  16bit boundary again. 

Example for 8bit RLE Compressed Data Expanded data 
03 04              04 04 04 
05 06              06 06 06 06 06 
00 03 45 56 67 00  45 56 67 
02 78              78 78 
00 02 05 01        Move 5 right and 1 up. (Windows docs say down,
                   which is wrong) 
00 00              End-of-line 
09 1E              1E 1E 1E 1E 1E 1E 1E 1E 1E 
00 01              End-of-bitmap 
00 00              Zero padding for 32bit boundary