dompdf is an HTML to PDF converter. At its heart, dompdf is (mostly) CSS2.1 compliant HTML layout and rendering engine written in PHP. It is a style-driven renderer: it will download and read external stylesheets, inline style tags, and the style attributes of individual HTML elements. It also supports most presentational HTML attributes.
PDF rendering is currently provided either by PDFLib (www.pdflib.com) or by a bundled version the R&OS CPDF class written by Wayne Munro (www.ros.co.nz/pdf). (Some performance related changes have been made to the R&OS class, however). In order to use PDFLib with dompdf, the PDFLib PECL extension is required. Using PDFLib improves performance and reduces the memory requirements of dompdf somewhat, while the R&OS CPDF class, though slightly slower, eliminates any dependencies on external PDF libraries.
Please note that dompdf works only with PHP 5. There are no plans for a PHP 4 port. If your web host does not offer PHP 5, I suggest either pestering them, or setting up your own PHP 5 box and using it to run dompdf. Your scripts on your web host can redirect PDF requests to your PHP 5 box.