Lenses come in different focal lengths, some have only one focal length such as 50mm, 105mm, 35mm. These lenses are called Primes and generally have low apertures such as f/1.8 or below.
Other lenses are called varifocal lenses, basically a lens which can zoom in and out (its focal length can be varied) These lenses come in many different focal lengths but the most common which comes with cameras is an 18-55mm lens, this means that its shortest (zoomed out) focal length is 18mm and its longest (zoomed in) focal lenth is 55mm. Also with lenses, the more that you zoom in (the longer the focal length) the depth of field decreases and you can get the same effect of a 50mm at f/2.8 even using a 200mm at f/8.
Sensor size is the size of your sensor compared to a 35mm full frame (standard for film) for example a the cheaper canon dslrs have a sensor called APS-C which basically means its smaller than the "full frame" by 1.6x. This 1.6x size difference means that the lenses on the camera arent actually the focal length that they state. An 18-55mm standard zoom on a full frame camera would be 18-55mm focal length, whereas on an APS-C sensor, the focal length is x by 1.6, therefore the focal length is actually 28.8-88mm.
The disadvantages of a smaller sensor are that if you want wide-angle shots, the lens you buy isnt as wide as you may wish, for example a 10mm fisheye is actually a 16mm and not as wide. The other disadvantage is that the quality sometimes isnt as good as a full frame sensor in low light. The advantage is that this extra "zoom" proves useful in many cases such as for sports photography and wildlife because it can save you alot of money on buying longer focal length lenses.
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