Xmlto is a front-end to an XSL toolchain. It chooses an appropriate stylesheet for the conversion you want and applies it using an external XSLT processor (currently, only xsltproc is supported). It also performs any necessary post-processing.
It supports converting from DocBook XML to DVI, XSL-FO, HTML and XHTML (one or multiple pages), epub, manual page, PDF, PostScript and plain text. It also supports converting from XSL-FO to DVI, PDF and PostScript.
DVI output requires dblatex or PassiveTeX. Other formats can be produced with any of the supported toolchains - dblatex, PassiveTeX or docbook-xsl/fop (but may require some extensions).