XSL isn't an entirely new specification. It is actually based on two existing specification languages: Cascading Style Sheets (CSS) and Document Style Semantics and Specification Language (DSSSL). CSS is a stylesheet language for HTML. It was designed primarily for describing the layout of web pages and as such often takes a "loose" approach to layout typical for of web documents. DSSSL is a Scheme-like language for formatting paged SGML documents. The influence of these two specifications is very apparent in the design of XSL. XSL is extremely derivative of DSSSL for its page layout scheme while it borrows heavily from CSS for formatting content within those pages.
The result of these influences is a language composed of three specifications:
XSL-FO is an XML application for directly describing how to render text on a page. XSLT is a data manipulation language that is intended to be used for converting regular XML documents into XSL-FO format. And Xpath is a small XML query language or addressing language that is used in conjunction with XSLT to perform data manipulation.
Although these three specifications together comprise the XSL specification, they can and are used separately, and in studying them, it is useful to look at them individually.