Lens basics

Some basic terms related to lenses are presented hereinafter. For more details, one is encouraged to consult the references listed at the bottom.

Prime vs. Zoom Lenses

Prime lenses have fixed focal length and don't zoom. Zoom lenses have the ability to zoom in or out and, when specified, both the minimum and the maximum focal lengths are indicated, e.g. 18-55 mm. This means that the focal length in zoom lenses can be adjusted continuously in the range from 18 mm to 55 mm, considering the previous example. Prime lenses tend to be smaller and lighter, have wider maximum apertures, and give sharper images than zoom lenses, but zoom lenses have become widespread to due their ability of producing different angles of view.

Wide-angle vs. Normal vs. Telephoto Lenses
Wide-angle lenses produce the widest angle of view, while telephoto lenses produce the narrowest. In between these two extremes stand up the normal lenses. In rough words, wide-angle lenses have focal lengths of 18 mm or shorter, while telephoto lenses are characterized by focal lengths of 55 mm or longer (assuming a sensor size of approx. 24 mm x 16 mm, available in most affordable SLR cameras).

Focal Length

Definition: It is the distance between the lens and the camera's sensor (or film) when the subject is in focus. It is usually stated in millimeters (e.g. a focal length of 55 mm).
Control: The focal length of a zoom lens can be adjusted rotating the lens' zoom ring. For a prime lens the focal length is fixed, i.e. the focal length can be changed only changing the camera lens.
Effects: A shorter focal length produces a wider angle of view and a large area of the scene is captured (apparent size of the subject is smaller). Conversely, a longer focal length leads to narrower angle of view and a small area of the scene is captured (hence the subject appears to be larger).

Tip: Due to the popularity of the 35 mm film standard (with size of 36 mm x 24 mm and diagonal of 43.3 mm) and today's availability of camera sensors with different sizes, camera-lens combinations may be described in terms of their 35 mm equivalent focal length for straightforward comparison. For example, a 55 mm focal length lens on a camera with sensor size of 22.3 mm x 14.9 mm (diagonal of 26.8 mm) has a 35 mm equivalent focal length of (55 mm) x (43.3 mm / 26.8 mm) = (55 mm) x 1.6 = 88 mm. Hence, a 88 mm focal length lens on a full-frame 35 mm camera will produce the same angle of view of a 55 mm focal length lens on a 22.3 mm x 14.9 mm sensor size camera.

Maximum Aperture
Definition: It is the maximum diameter of the lens opening that can be selected with the lens. The maximum aperture is achieved with the minimum f-stop number of the lens.
Control: The maximum aperture is a lens specification and the photographer cannot change it unless another lens is used. This specification is generally included in the lens model name (e.g. the zoom lens Canon EF-S 18-55mm f/3.5-5.6 IS II has maximum apertures of f/3.5 when at focus length of 18 mm and of f/5.6 when at focus length of 55 mm).
Effects: Lenses with wider maximum apertures (lower minimum f-stop numbers) create a brighter image on the camera's sensor than lenses with small maximum apertures (higher minimum f-stop numbers). Also, wider maximum apertures allow smaller depths of field than narrower maximum apertures do.

Minimum Focus Distance
Definition: It is the shortest distance from the focal plane mark on the camera body to the subject at which a lens can still focus.
Control: The minimum focus distance is a lens specification (usually included in the model name) and the photographer cannot change it unless another lens is used (or an accessory is connect to the lens).
Effects: If the camera is too close to the subject that the minimum focus distance is violated, then to put the subject in focus will be very difficult or even impossible.

Angle of View
Definition: It is the visible extend of the scene captured by the image sensor.
Control: The angle of view is indirectly controlled by the lens' focal length.
Effects: A wider angle of view captures a greater area, while a narrower angle of view captures a smaller area of the scene.

References:
[1] Nikon Website, Digital SLR Camera Basics - Lens
[2] Andy Westlake (dpreview), Digital camera lens buying guide

[3] Ed Grabianowski, 10 Important Photography Terms - Lens
[4] Wikipedia, 35 mm equivalent focal length
[5] Max Lyons, Photography Calculators