This is a very broad question, and I'm tempted to suggest that you buy and read Scott Kelby's Digital Photography book series. It's not the most technical series, but it gives you a nice introduction and tips for taking photos.

As for the more concrete question:

ISO was (is) the sensitivity of the film in a film SLR. In digital SLRs it's the analogue and digital amplification in the sensor. It's a number that describes how much light the film/sensor need to correctly expose an image. A low ISO will require more light. If you increase the ISO you can have a smaller aperture and shorter shutter time, and still get a correct exposure.

Aperture is the 'size of the hole' in the lens. This controls how much light that comes through the lens. A large aperture (denoted with a small f-number) will let more light through the lens. The aperture is also responsible for changing the amount of bokeh in an image. Larger aperture means you can use lower ISO and shorter shutter times.

Focal length translates into field of view for a given camera. A long focal length will give a narrow field of view, while a short focal length will give a wide field of view. A short focal length will be similar to 'zooming out' on a compact camera, i.e. you can capture more of a scene.

One important variable you forgot to ask about is shutter speed. This is the length of time of the exposure. A fast shutter speed (e.g. 1/250th of a second) will 'freeze' the picture, while a longer (1/5) will give you motion blur in the image. By increasing the shutter time, you can use a smaller aperture or a lower ISO setting.

To get the correct exposure you need to balance ISO, aperture and shutter speed. Most cameras will do this automatically, but with a SLR you can normally also have manual control over one or more of these variables. This gives you more control and "artistic" freedom.