Possibly the most important piece of any camera, a quality lens can take an image to "okay" to stunning. As with all of photography though, equipment can only take you so far. Selecting a lens will depend greatly on the subject. Lenses mainly fall into two categories. Prime lenses with a single focal length or Zoom lenses with varying focal lengths.
Prime Lenses: Prime lenses, are naturally less complicated than their zoom counterparts. As such, they're usually sharper, less expensive and have much larger apertures. Ever photographer should have a 50mm (Full Frame) or 35mm (Crop Sensor) lens in their bag.
Pros: Price, simplicity, sharpness, common apertures for most primes are f/1.8 or faster (larger apertures). Lighter & more compact. Ability to create stunning background separation/"bokeh"/background blur.
Cons: Having to switch lenses. Can be bulky if you take multiple lenses. Single focal length means having to walk around to achieve your desired composition.
Zoom Lenses: Zoom lenses are the standard for most D-SLR kits. The common focal lengths range from around 18-55mm for crop sensor cameras. Kit lenses & lower end zooms however, usually have a varied aperture. For the Nikon 18-55mm kit lens for example, it has a maximum aperture of f/3.5 at 18mm (wide angle) and down to f/5.5 at 55mm(medium telephoto). It's a sharp lens in when uses outdoors in bright sunlight, but at f/5.5, it's pretty much useless for handheld shots at 55mm indoors. You'll either need to boost the ISO, which creates noise or steady the camera for longer exposures. You can use a flash, but that's a whole other topic with pros and cons. Professional zooms have large apertures, but expect to pay significantly more. They also tend to be a lot heavier, bulkier & overall larger.
Pros: Multiple lenses in one package. Easier to compose a shot. Little to no lens switching required.
Cons: Usually slower apertures for consumer level lenses. Pro grade zooms are very expensive. Not as sharp, although most or of the recent offerings have gotten quite good.
Image Stabilization: Offered on some lenses, these systems reduce vibrations. Although handy for most handheld situations, this should be turned off for any shot where a tripod is available. It's an effective tool for telephoto focal lengths.
There are tons of info scattered all over the web regarding lenses and way to much information to cover in a single post. The best choice will be dictated by the type of photography you're most interested in. If you have any questions, feel free to leave a comment.
