..if you are not buying a bundle lens..read on.

Almost everyone wants to have a lens that would serve all purpose...an all in one..but sadly such lens don't exist and even if they do they are too highly priced.

Lens should be purchased based on the kind of pictures one would like to take Landscapes , Portraits , Sports, Wildlife or Macro Photography. Each of these types need a lens of a different focal length.

Besides the focal length , another important aspect is the maximum aperture that the lens has.Aperture controls the amount of light that passes in through the lens. A wide aperture is better , preferable and more expensive.

Lenses can be classified as Prime or Zoom.
Prime lens have a fixed focal length and one needs to move up and down to get closer to the subject while zooms have a variable focal length.Prime lenses give better picture quality , are lighter and comparatively much cheaper than zoom lenses.

Zoom lens have variable aperture , while lens with constant aperture are also available but are comparatively expensive.

Some example of specifications of the lens are :

50mm f/1.8 - a prime lens with 50mm focal length and max aperture of 1.8
18-55mm f/3.5-5.6 - a zoom lens focal length 18-33mm and also variable max aperture
17-50mm f/2.8 - a zoom lens with focal length 17-50 mm and constant max aperture of 2.8

The f/2.8 has a wider maximum aperture value than f/3.5- 5.6. A wide max aperture value comes handy when taking pictures in low light and while capturing moving objects.

The range of focal length and the best use are as below:

Lens Type - Best Use - Focal length

Wide Angle - Landscapes and Interiors - f28mm or lower
Standard - Portraits - f35mm to 85mm
Telephoto - Portraits and Sports - f 100 - 300mm
Super-Telephoto - Wildlife and Sports - f 300mm and higher

Third party lenses : Lenses are manufactured by Canon , Nikon and also by third party manufacturers like Tamron , Sigma, Tokina and many more.
Third party lenses are good alternatives for people on budget as the third party manufacturers have come a long way and provide equally good lenses but at a modest price.

I am mostly interested in Landscapes, Portraits.The lens that I have with me is the Tamron 17-50 mm/f 2.8.I have very happy with my purchase as it also has Image stabilization which takes care of shake if the hand is not steady.My next purchase would be a Telephoto lens.

I read a lot of reviews before deciding on the lens.Here are few sites I refer a lot to.
Indepth understanding of lenses
Lens Reviews
Dp Reviews
Tamron lens reviews

The following parameters would help to shortlist the right lens example:
Lens mount: SLR Make eg Canon/ Nikon
Lens focal length: Telephoto/ Wide/ Standard
Maximum aperture: f/2.8 - f/3.5
Zoom aperture: Constant/ variable
First vs third party: Canon/ Tamron
Extra features: Image stabilization etc.