One of the most important explanatory principles behind online businesses is the Power Law, defined by Wikipedia as “any polynomial relationship that exhibits the property of scale invariance“. One more clear embodiment of the same law is the well-known “80-20 rule”, also known as Pareto principle - which Wikipedia clarifies by explaining that “80% of the effects comes from 20% of the causes”. The following graph probably looks familiar.

The shape differs sharply from bell curve a.k.a. normal distribution, which rules in the offline world. Most of the natural things are distributed “normally” - meaning that there are many average things, and few extremes.
However, because of the Internet’s linked nature and free distribution of information in Internet, many things in online business are not distributed by the bell curve, but instead according to the power law, where there is few extremely big things, sharply declining number of smaller things and a long tail of small things. You get the idea from the graph above.
When considering websites, this rule of thumb would mean that 20% of websites attract 80% of traffic. Sounds very beliveable, and I would even expect that the figures would be even more sharper - let’s say that 5% of websites attract 90% of traffic (the numbers don’t have to sum to 100, they measure different things). What might be Google’s share of all web traffic?
One consequence of the same law is that “rich will get richer” and “winner takes it all”. The big ones tend to grow even bigger. We can see this in effect in for example online auctions or social networks. It doesn’t make sense to sell something on a website in which there are only few other visitors - so every rational person should sell his stuff on the biggest possible site. On each market, there usually is one clearly leading online consumer marketplace.
The downside to this is that without specialization only the big online businesses will be viable. There are no viable middle-sized portals and there are no small generalist online marketplaces. Everyone has to try to get the network effect to work on their behalf - or lose the game. Unless you can find a niche.
This leads us to other use of the same graph, that of the long tail. Thanks to the lowering distribution (and inventory) costs made possible by Internet, “the endless choice is creating unlimited demand” (Chris Anderson). Internet has opened markets for such niche products that could not have existed earlier - and to those that existed, it has opened completely new markets and possibilities for growth. Some have even talked about slow death of mass media.
What is interesting about the long tail is that the volume of the tail can be as big, or even bigger than the volume of the head. So curiosities and niches combined create a considerable market.
But it’s the same law, which has created these two opposite phenomenons. There is no market for middle-sized “department stores” on the web, because everyone will go for the huge ones - simply because they are the best. However, there is market for small specialized stores, concentrating on a chosen niche. And there is an endless market for all kinds of cultural products, which can find their audience through the web. But the networks they must use to find their audiences must be the big ones - because everyone is there.
To sum up: if you can’t be the biggest in your market, you must find a viable niche.