What is the Web?
The Web, or more properly the World Wide Web, is part of the Internet. The Internet is a worldwide collection of computer networks that exchange data by using common standards called protocols. The Web is a shared space on the Internet that allows computers to use a common protocol, Hypertext Transfer Protocol (HTTP), to access webpages via hyperlinks. Hyperlinks are direct links to another web page. They can be embedded in the text and by clicking on the link the user is taken to the new page. A web browser such as Microsoft Internet Explorer, Mozilla Firefox or Google Chrome is a software application that we use to interact with webservers and access documents on the Web.
Some usage characteristics
It is hard to know how large the Web is. One estimate suggests that it has over 600 billion indexable pages.  Comscore reports 18.6 billion searches in February 2013 for the US alone.  There are likely to be well over the 100,000 health websites estimated in 2002. 
Eysenbach and Kohler  have suggested that 4.5% of internet searches are health related. Given that Google reports over 1 billion searches per day, this represents at least 45 million health related searches every day on Google alone.
Considerations in using the Web
There are things to consider when searching the Web. Firstly, the Web is an unregulated environment. Provided you can access the internet, it is possible to create a website. Unlike journals, websites and webpages may not have a peer review process. As there is no requirement to maintain a site, the information may be outdated. Material is often transient in that it can be moved or removed without notice, effectively ceasing to exist.
There are two drawbacks in using search engines on the Web. First, they do not directly assess the quality of sites and hence the materials retrieved. Second, searching is based on textwords rather than a library indexing system such as MeSH terms. This means that a search term may return many irrelevant items. Web searches can also return thousands of possible matches.
However, there are, many benefits. The Web contains information and materials that may not be able to be accessed in any other way. It enables access to and sharing of databases and datasets. There are also new applications for use in the web environment that can facilitate online dissemination and interaction  such as social bookmarking systems (e.g. delicious and diigo) or social citations (CiteULike).