The AVHRR sensor scans the earth below and transmits what it sees continuously. It views the earth in 5 wavelength channels, from this data images can be made. The images on the pages here fall into two types: color composites and channel 3 images.
Color composite images use 3 channels of the AVHRR imager on the satellite. Channel 1 is in the visible range and is colored red, channel 2 is in the near infrared and is colored green, channel 4 is in the far infrared is and colored blue after being reversed. The peak reflectance from vegetation falls in channel 2, so the green color does represent vegetation mostly. Channel 4 shows the thermal emission, warmer areas emit more, but this channel has been reversed to make clouds bright instead of dark. The color composite images show smoke plumes as yellow or yellow/green in color. This is because the smoke only shows up in channels 1 (visible) and 2 (near IR), not in channel 4 (far IR) which cuts through the smoke. Since channels 1 and 2 are colored red and green, and red and green light when combined makes yellow, the smoke shows as yellow. Note that low warm clouds (also fog) may show as yellow since they are dark in the reversed channel 4 (so have little blue to their color).
Channel 3 images cut through the smoke and show the fires themselves as bright areas. The deep black areas are clouds which are colder and darker than the warm earth below. Channel 3 images commonly show smaller fires than the color composite smoke plume image. The nature of the channel 3 images varies between day and night images. On the daylight images channel shows a mixture of reflected sunlight and thermal emission from the scene below. Some of the brighter edges of the clouds may be bright on the channel 3 daytime images. At night channel 3 shows only the thermal emission. Variations in water temperature may be seen in the gulf and off the east coast. Also note that as the land warms and cools with the day/night cycle the contrast with the lakes and oceans will vary. During the day land may be a bit brighter or about the same as water areas, at night the land may be darker since it cools faster than the water.
With a bit of practice it is possible to see a lot of details in these images.