directory This of this is a giant yellow pages site containing an index of other relevant sites or services. An example is DMOZ or the Yahoo directory.
dofollow - Dofollow usually forms part of a link expression. It is inserted by plugins such as CommentLuv to ensure your blog gives commentors link love.
doorway - Also known as a gateway. This is another bad, bad thing to avoid doing - it is a web page that is designed specifically to attract traffic from a search engine. A doorway page which redirects users (but not spiders) to another site or page is implementing cloaking.
duplicate content - Also see canonical issues earlier. Obviously content which is similar or identical to that found on another website or page. You may not suffer from hosting duplicate content but also don't expect to gain any authority based on it. Be original in your content creation
e commerce - this is a website devoted to commercial sales like Amazon for example.