Bot is derived from robot and stands for a at least somewhat autonomous computer program.
According to Wikipedia's Software agent article, “In computer science, a software agent is a computer program that acts for a user or other program in a relationship of agency, which derives from the Latin agere (to do): an agreement to act on one's behalf. Such "action on behalf of" implies the authority to decide which, if any, action is appropriate. Agents are colloquially known as bots, from robot. They may be embodied, as when execution is paired with a robot body, or as software such as a chatbot executing on a phone (e.g. Siri) or other computing device. Software Agents may be autonomous or work together with other agents or people. Software agents interacting with people (e.g. chatbots, human-robot interaction environments) may possess human-like qualities such as natural language understanding and speech, personality or embody humanoid form (see Asimo).” (Aug. 2017)
Wikipedia defines Social bot (also: socialbot or socbot) as “a particular type of chatterbot that is employed in social media networks to automatically generate messages (e.g. tweets) or in general advocate certain ideas, support campaigns, and public relations either by acting as a "follower" or even as a fake account that gathers followers itself. In this respect, social bots can be said to have passed the Turing test.” (Aug. 2017)
“An Internet bot, also known as web robot, WWW robot or simply bot, is a software application that runs automated tasks (scripts) over the Internet. Typically, bots perform tasks that are both simple and structurally repetitive, at a much higher rate than would be possible for a human alone. The largest use of bots is in web spidering (web crawler), in which an automated script fetches, analyzes and files information from web servers at many times the speed of a human. More than half of all web traffic is made up of bots.” (Internet bot (Wikipedia, Aug. 2017))