A robot is a program that automatically traverses the Web's hypertext structure by retrieving a document, and recursively retrieving all documents that are referenced.
Note that "recursive" here doesn't limit the definition to any specific traversal algorithm; even if a robot applies some heuristic to the selection and order of documents to visit and spaces out requests over a long space of time, it is still a robot.
Normal Web browsers are not robots, because they are operated by a human, and don't automatically retrieve referenced documents (other than inline images).
Web robots are sometimes referred to as Web Wanderers, Web Crawlers, or Spiders. These names are a bit misleading as they give the impression the software itself moves between sites like a virus; this not the case, a robot simply visits sites by requesting documents from them.