From Wikipedia:
A style sheet language, or style language, is a computer language that expresses the presentation of structured documents. One attractive feature of structured documents is that the content can be reused in many contexts and presented in various ways. Different style sheets can be attached to the logical structure to produce different presentations.

One modern style sheet language with widespread use is Cascading Style Sheets (CSS), which is used to style documents written in HTML, XHTML, SVG, XUL, and other markup languages.

For content in structured documents to be presented, a set of stylistic rules – describing, for example, colors, fonts and layout – must be applied. A collection of stylistic rules is called a style sheet. Style sheets in the form of written documents have a long history of use by editors and typographers to ensure consistency of presentation, spelling and punctuation. In electronic publishing, style sheet languages are mostly used in the context of visual presentation rather than spelling and punctuation.

The following is a list of style sheet languages.

Cascading Style Sheets (CSS)
Document Style Semantics and Specification Language (DSSSL)
Extensible Stylesheet Language (XSL)

JavaScript Style Sheets (JSSS)
Formatted Output Specification Instance (FOSI)
Syntactically Awesome Stylesheets (Sass)
SMIL Timesheets