Robot - A robot is a program that runs automatically without human intervention. Typically, a robot is endowed with basic logic so that it can react to different situations it may encounter. One common type of robot is a content-indexing spider, or webcrawler.

Spider - A spider is an automated program that "crawls" the Web, generally for the purpose of indexing web pages for use by search engines. Because most web pages contain links to other pages, a spider can start almost anywhere. As soon as it sees a link to another page, it goes off and fetches it. Large search engines have many spiders working in parallel.