About Camera Lenses
Camera lens is an optical lens which when used with a camera body creates images. Basically, main purpose of lens is to bend/concentrate light rays so that digital sensor can record the scene.
There are two main aspects to every lens, so called parameters: focal length and aperture.
Focal Length determines angle of view: Short (wide angle of view) and Long (telephoto).
Here is the rough estimate on how lenses are used according to their focal lengths:
Less than 21 mm - Extreme Wide Angle - Architecture
21-35 mm - Wide Angle - Landscape
35-70 mm - Normal - Street & Documentary
70-135 mm - Medium Telephoto - Portraiture
135-300+ mm - Telephoto - Sports, Bird, Wildlife
For example, when shooting landscapes, wide angle lenses are the best because you can focus on a very close subject and still have a lot of background in your image. Wide angle will allow you to use a tree, rock, shrub to create a better composition and with that create a 3D illusion, a sense of great depth.
Or, when shooting portraits medium Telephoto lens is recommended because you can get closer without compromising shooting distance and perspective. It is also considered to be more flattering because it doesn't exaggerate features such as noses.
Aperture is a mechanism in a lens which can be opened or closed down to control the amount of light going through. Aperture controls the depth of field. If it's opened at maximum, it will give a max narrow depth of field. Aperture is specified as f-stops. It kind of works opposite, smaller the f-stop, larger the opening. Quality of your lens depends on the maximum aperture it can achieve. To put it simply, when buying a lens try to get the one with the smallest f-stop capability.
Smaller f-stop = Larger the Opening/Aperture = Narrower Depth of Field
Zoom Lenses are great but...
Zoom lens lets you change focal lengths without changing lenses but they tend to be much slower. They admit less light than prime lenses. Longer the zoom range, the slower they can be. If you would like a relatively fast zoom lens (i.e, f/2.8), it will cost you. Such lenses tend to be pricey.
Another drawback of zoom lenses is that it can limit one's perception and creativity. When using zoom lenses it is easy to forget your feet and not move around. If you do so, you will lose out on different possibilities and perspectives.
When choosing your lens, consider focal length and it's maximum aperture (smallest f-stop) capability. Choose focal length according to what you like to shoot, and maximum aperture according to your budget. Thanks for reading.