In Internet terminology, a bot, also known as a crawler or a spider is an automated or semi-automated computer program which accesses Internet resources.
Bots can be used for a variety of purposes, search-engines are dependent on bots to index web-pages, this is the most popular application of Internet bots, indeed: so strong was the connection between the term "bot" and search engines that a search engine named "HotBot" was launched in 1996, it was not a success.
Bots can also be used for criminal purposes, such as harvesting email address, phone numbers and other identifying information for undesirable purposes, bots can also be used to mount attacks on web-sites by accessing them en-masse in an attempt to provoke a server crash.
Technology used to combat bots typically involves requiring the input of information that only a human could know, such as requiring the user to decode distorted text, answer a math problem or enter common knowledge. These techniques are controversial as they sometimes impede the disabled and may frustrate users if they are too difficult.