Here's a good reference:
Link
On order to understand f-stop, you first need to learn the "scale". Just remember the numbers 1 and 1.4. The scale sequence takes the f-stop two positions back and doubles it. So given 1 and 1.4 are your starting values, the stops on the f-stop scale are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11 (round), 16, 22, 32... These numbers are a logrithmic measure of the amount of availabe light that passes through the lens. A move one stop down the list cuts the amount of light passing through the lens by half. A move of two stops (say f4 to f8) cuts the light by 75% (half as much, and then half as much again). Each move up the scale doubles the amount of light. A move of two stops up (say f5.6 to f2.8) is four times as much light.
Each lens has a maximum possible "f-stop" (AKA "aperture") based on the design of the lens and the quality of the glass used(*). That aperture can be adjusted downward using the lens "iris"... just like the human eye. Depth of Field is a characteristic whereby the range of distances that appear "in focus" is dependand on the focal length of the lens and the f-stop it is set at. The smaller the f-stop, the greater the DOF... and vice versa. The DOF is the shallowest for any lens with the aperture is fully open. If you wear glasses and are near or far sighted, there's a great excerise to illustrate the DOF concept. Take you glasses off and look at an object that's out of focus to your un-aided eyes. Close one eye, bend an index finger down and hold it closed with your thumb so there's a small "pin hole" at the center of your closed finger, now look through the "pin hole" with your open eye... you'll notice that the out of focus object is now probably a lot more in focus! That's DOF at work!
* = This is a primary reason that two lenses from the same maker may vary greatly in price.... One may have a maximum aperture that's a stop or two higher than the cheaper lens. A lens with a maximum aperture of f2.8 (vs. f5.6) is usable in a lot more lighting situations.
