Nikon attempted to reduce the price of consumer and upper entry level DSLR’s in late 07 which took away the in built auto focus system. Now that upset a lot of customers because Older lenses could no longer be auto focused and the Nikkor lenses were and are still quiet expensive.They countered the criticism by introducing the AF-S lenses with the VR facility in it.
This ultimately led to the boom of 3rd party lenses.Sigma and Tamron rank almost equally high in quality and they are reasonably priced. Both are Japanese companies, established in the early 50’s and 60’s. Both manufacturers produce 70-200 mm as well as 70-300mm AF-S lenses which rose to fame due to its capacity to deliver crisp quality images at say, half the price. Apart from that they both manufacture higher end lenses, which are of incomparable in quality and quiet unreachable in price for most amateur photographers (especially Sigma). Today a Tamron 70-300mm 4-5.6 AF-S Macro lens comes at around Rs. 8500.00 . As compared to Nikon’s 70-300 mm which costs approximately Rs. 16500.00 .
Now, you might be wondering why the 70-300 mm lenses are priced so low. For starters they lack the feature of the built in Image Stabilization, or in Nikon’s terminology the VR feature.( keep the price in mind and a tripod sorts the vibration out ) But Tamron (I’m not sure of Sigma) comes with a built in AF system, enabling it to focus without the aid of a built in motor (in the body). Both shoot razor sharp images till about 200 mm, but as you move further you start to notice color fringing, especially in images with white in it. Besides the built quality is not all that great, both feel as if it was made from cheap plastic. They are also a lot noisier, although Tamron has come out with a , yet to be tested, low sound version of the 70-300 mm lens.
To conclude we have two traditional lens manufacturers Sigma, Tamron (Quantaray, another manufacturer, uses the same lens as Tamron) giving you cheap lenses of 70-300mm with built in AF-systems and good image quality up to, say, 200-220 mm, after which you start to notice color fringing.
I hope the above description sorted out your doubts. please feel to ask any further doubts.