The core basics for photography include ISO, shutter speed and aperture. That trifecta isn't all that makes up photography though, focal length will also factor greatly into your photography and have a direct impact upon your final image. Wikipedia describes photography focal length as:
longer focal length or lower optical power is associated with larger magnification of distant objects, and a narrower angle of view. Conversely, shorter focal length or higher optical power is associated with a wider angle of view.
I'll be the first to admit that is pretty confusing. Essentially, focal length, based off that explanation is how much your lens will be able to zoom into the view, but doing so will create a narrower angle of view. The best way to explain this concept is with a photograph:
How this affects your photography after looking at the above example starts to become very apparent, and your choice of lenses for any given situation becomes even more important.
While there is no perfect lens for any given situation, there are preferred lenses. For example, portrait photographers generally prefer lenses in the 85-200mm range, and Canon's 135 f/2L lens is among one of their favorites. This is a stunning example, shot at f/2:
The longer focal length here, combined with the wide open aperture creates a very distinct depth of field, or blurred out section behind the child's face. Additionally, this focal length is chosen by portrait shooters because it tends to be the most forgiving for human defects, such as longer noses or larger necks. The downside to shooting these longer focal lengths is that a very fast shutter speed must be used, one at least as fast as the focal length. This shooter used a 1/200th shot to freeze the action.
Landscape photographers often prefer using wide angle lenses to capture the full field of view, and this stunning photograph is a prime example:
Shot with an 11-16mm lens @ 15mm, on a 1.5x crop sensor Nikon, the effective focal length of this capture is 22.5mm, still very wide. Unlike the longer lenses, which will create great depth of field when shot open, wide angle lenses simple don't have the ability to do so, and this being shot at f/4 shows that. This is because of the focal length relationship to the subject, and the sensor in the camera. Again, for most of us, it's hogwash math, but knowing the effects is vital information. The upside to shooting wides is that a much shorter shutter speed needs to be used to freeze action.
When selecting the lens you want for any given circumstance you'll have to evaluate what you're trying to accomplish. Most wildlife and sports photographers prefer to use long focal length lenses, some to 600mm to get close to the action, architect and landscape photographers usually prefer wide lenses, all the way to 14mm in some cases. Fashion, portrait and even family photographers usually fall within the 50-200mm range. If you shoot any combination of these, clearly you can see there won't be just one single lens to fit your needs, rather a few pieces.