First class started slowly, allowing us to get into the rythm of things again.
After introducing ourselves to our renewed class group, it was brushuptime:

Aperture (the larger the aperture opening, the LOWER the number):

1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32 - 45 - 64 - 90 - 128

Shutter Speed (faster to slower):
1/8000 - 1/1000 - 1/500 - 1/250 - 1/125 - 1/60 - 1/30 - 1/15 - 1/8 - 1/4 - 1/2

DON'T forget your camera only indicates the denominator ... so you will see 8000, 1000, 500, 250, 125, 60, 30, 15, 8, 4 and 2

continuing: 1 - 2 - 4 - 8 - 16 - 30 - bulb
seconds will be indicated with ".

All these numbers represent 1 STOP.
And 1 stop is a doubling or a halving of the amount of light which falls into your sensor.

So with the aperature and shutter speed you will control the exposure of your sensor !

The ISO sets the light sensitivity of your sensor (as with the analogue cameras you would buy different film). ISO also represent 1 STOP per jump:

25 - 50 - 100 - 200 - 400 - 800 - 1600 - 3200 - 6400 - 12800 (max for my EOS 50d) - 25600 (max for the EOS 5d mark II).

The higher the number, the more light sensitve the sensor will be (electronically that is, in reality the sensore remains the same). The higher the ISO, the higher the noise (see examples).

Changing the aperture will influence the DOF, the depth of field. The larger the aperture (smaller number), the shorter the DOF.
Changing the shutter speed will influence your photo in another way: short shutter speed will freeze your image, the longer the shutter speed, the more motion the photo will show. This can be a conscious choice OR result in blur pics.
NOT using a tripod, 1/50 or 1/30 (indicated 50 or 30) would be the longest shutter speed to use.