A web-dl is for cases where the stream encoding has been cracked or whatever so the torrent is identical (minus any drm) to the content from the web store. Itunes DRM has been cracked so that is a typical example of this (look up requiem).
A webrip is a capture using a capture card or even just screencapping software (in extreme cases) to capture from internet streams like netflix (or often dodgy korean sites) so there will be encoding artifacts, streaming artifacts, quality limitations and sometimes hardcoded subtitles.
简单的说就是，WEBRip 是经过转码的，而 WEB-DL 未经过转码。
