Focal length is one of the simpler ones. It is the distance from the end of the optical centre of the lens to the focal plane. So the middle of the lens to the sensor. The focal plane is usually marked on the camera body by a cicle with a horizontal line on it. A longer focal length has a smaller field of view, and brings you closer to the subject, while a short focal length has a wider field of view and brings you further away from the subject. The human eye has a focal length roughly equivalent to 50mm - this isn't that accurate, but looking through a viewfinder with a 50mm lens, objects are around the same size as when seen with the naked eye. Lenses wider (shorter focal length) than 50mm can see more of a subject when close to it. Wide angle lenses can lead to quite dramatic effects, as they can cause curving of straight objects close to the focal plane, as well as allowing for a large field of view, meaning more gets in the frame.

Cameras using an APS-C sensor (Canon xxxD, xxD) are subject to a 1.6x crop factor, giving lenses effectively 1.6x the focal length. Cameras using an APS-H sensor (Canon 1D) have a 1.3x crop factor, with an equal 1.3x effective focal lens. Cameras using a full-frame 35mm sensor (Canon 1Ds, 5D) have no crop factor. This means that crop-sensor bodies have a longer reach at the telephoto end (150mm+), as the long end is effectively 'longer', giving them a reach advantage over full-frame. However, full-frame bodies with no crop factor are more effective at wide angles, because equally on a crop-sensor, short focal lengths are multiplied. 17mm on a full-frame gives a very wide image, but on a 1.6x crop-body has an effective focal length of 27mm, leading to a smaller field of view.

Focal length has an effect on Bokeh;

Shorter focal lengths (wider angle) produce a larger depth of field and a closer hyperfocal distance, leading to a less pronounced bokeh, with more of the scene in focus (assuming the scene constitutes of objects with varying distances to the camera).

Longer focal lengths (telephoto) produce a narrower depth of field and a further hyperfocal distance, leading to more pronounced bokeh, with less of the scene in focus (assuming the scene constitutes of objects with varying distances to the camera).

Focal length also has an effect on usable Shutter Speed;

Shorter focal lengths have a shorter distance from the optical centre to the focal plane and as such, are less succeptible to camera shake, reducing the required acceptable shutter speed for a sharp image, and allowing more light in, enabling lower-light shooting.

Longer focal lengths have a longer distance from the optical centre to the focal plane, and such exaggerate camera shake, increasing the required acceptable shutter speed for a sharp image, requiring more available light.

A general rule of thumb for camera shake is that an acceptable shutter speed is 1/focal length. A 15mm lens can be handheld easily enough at 1/15s exposure without noticable camera shake, but a 200mm lens needs a faster shutter speed, 1/200 to have no noticable camera shake. The better your posture and ability to hold a camera still reduces these requirements, and equally, some people find it hard to handhold for longer exposures, so these numbers will vary. Image Stabilisation mechanisms present in some lenses (or for certain Sony and Olympus DSLRs, in the body) allow using a shutter speed two or three stops slower than normally required, enabling lower-light shooting. These lenses are denoted by 'IS' (Image Stabilisation) in the lens name.