It's impossible to be absolutely certain how many there are, but Netcraft released their latest (December 2012) survey of web servers and concluded there are about 633 million websites in the world today – a rise of 78 million (28%) in the last year.
The number of active sites is 186 million. The inactive sites are made up domain names that are registered, but unused. For example, a company might purchase several domain names that all direct you to the same website. Most of us use the internet, but few understand the complexities of what's going on behind the scenes of any website. There's a lot going on, and you can build a pretty do-it-yourself site, but without at least some basic knowledge it will never be found on the world wide web.
A small local business does not need to spend hundreds or thousands of dollars optimizing a site to attract traffic from all over the world.
The focus or purpose is to create a positive image on the web that acts as an advertisement and offers a convenient way for your existing and potential customers to access more information about your business.
I see websites in one of two categories. National/Global or Local. The goal for each is different and so the sites themselves and the platform, or software used to build the sites are as well. The national or global sites break down into several more categories, some examples would be big business (Apple), social networking (Facebook), large content sites that focus on a niche and make money through advertising for related companies.(Amazing Olive Oil).
The main differences being size and SEO (search engine optimization) Basic SEO is important to any website, but with large sites it's #1 priority is bringing traffic to the site.