The asterisk has many usages. Most operating systems treat * as a wild card that refers to "all files" when it appears in a command line.
Online, the asterisk is used primarily in search engines as part of search strings, where placing * at the end of a keyword will broaden your search. For example, "market*" will find results with market, marketing, marketplace, marketer, markets, etc.
Also used in e-mail or chat rooms when someone want to emphasize something they've said, stars may appear in the middle of a sentence, such as "I'm so *happy* about that!" In online learning environments, a pair of asterisks may also be placed around words to denote action, such as *yawns* or *checks paper for mistakes.*
The asterisk is also used to censor an obscene word by replacing a letter, for example, "That sounds like pure bull sh** if you ask me." You will see A LOT of asterisks replacing letters of the Chat Acronyms and Text Messaging Shorthand page!