The same difference between purling a stitch and knitting a stitch. They both twist the st, though you may not see it on the purl side. The back of a knit looks like a purl, the back of a purl looks like a knit. It's the same stitch, but it depends on which side you make it and how you insert your needle. Have you seen the videos for knit and purl here? And there's also videos for doing them tbl on the Glossary page.
sue- knitting heretic