A zoom lens has a variable focal length, for example 18-55mm, meaning a wide-angle 18mm frame can be shot followed by re-framing using the zoom at, say, 48mm or 55mm - and all without so much as moving from the same spot.
This reduces the need to swap between lenses when shooting, with some zoom lenses covering such a wide range there may be little reason to ever change lens.
On the downside, the more basic zooms such as Panasonic's 14-42mm f/3.5-5.6 Micro Four Thirds lens often have limited aperture control, in this case no brighter than f/3.5 at the wide end and, due to the optical constraints, even less bright at f/5.6 at the long 42mm end of the zoom.
Fixed aperture zooms are available, but these are often far larger, heavier and considerably more expensive.
Kit lenses are basic zoom lenses most frequently boxed up with a camera body at the point of purchase. Micro Four Thirds cameras tend to come with 14-42mm optics, while most APS-C DSLRs come with 18-55mm lenses.
In recent years, more advanced models have often come boxed up with more versatile lenses, such as the Nikon 18-105mm f/3.5-5.6 coming with the Nikon D7000.
Optical constraints and moving lens parts often mean image quality and sharpness from zoom lenses may be at its best at a particular focal length - such as (hypothetically) an 18-55mm lens producing its sharpest images from 28-40mm.
Often a zoom's sharpness won't match up to a prime lens, which is among their bigger downsides. If you want that extra lick of optical quality and greater aperture control through a similar focal length range, then lenses such as Tamron's 28-75mm offers a fixed f/2.8 wide aperture at all focal lengths and is a great step-up option to replace a more basic kit lens.
Typical kit or standard zoom lenses
Canon EF-S 15-85mm f/3.5-5.6 IS USM - £609
Nikon Nikkor 18-105mm AF-S DX f/3.5-5.6G ED VR - £225
Sony A-mount 28-75mm f/2.8 SAM - £629
Pentax SMC DA 18-135mm f/3.5-5.6 ED AL IF DC WR
Tamron SP AF 28-75mm f/2.8 XR Di - £358
Panasonic Lumix G Vario 14-42mm f/3.5-5.6 Micro Four Thirds - £124
Samsung NX 20-50mm f/3.5-5.6 ED - £153
Sony E-mount (NEX) 18-55mm f/3.5-5.6 OSS - £240