One of the cool features of JPEG files is that there's the option to save low frequency components first. This means that a partially downloaded image will be displayed at low resolution instead of being cut off.
In the file, this works by breaking up the compressed data into multiple "scans", each prefixed with a header. Here's the first scan of a representive image:
... this one includes the lowest (DC) Fourier bin for all three color channels.
The three color channels are YCbCr instead of the usual RGB. The luminance (Y) seperated because it must be high quality, but the color can be fudged quite a bit while looking fine. Very roughly: Y = G, Cb = B - G, Cr = R - G
After it, the file contains eight more scans to fill in the rest of the data:
Scan number Channels DCT bin range Precision 0 Y Cb Cr 0 - 0 Half (-1 bit) 1 Y 1 - 5 Quarter (-2 bits) 2 Cb 1 - 63 Half 3 Cr 1 - 63 Half 4 Y 6 - 63 Quarter 5 Y 1 - 63 Half 6 Y Cr Cb 0 - 0 Full 7 Cr 1 - 63 Full 8 Cb 1 - 63 Full 9 Y 1 - 63 Full