The main difference is tags are single words with no proper meaning like "SURFING" "VIDEO" keywords give some exact meaning like "SURFING VIDEO"
Keywords are fundamentally the themes you are going over on your website. Meta tags are hidden fields utilized to work search engines interpret what your website is around so they can engage visitors with results that are the closest to what they are searching for.
keywords are the phrases people search for. You need to optimize your articles for keywords. Tags are sometimes used as keywords to increase the keyword density. Also in wordpress a tag can create a special tags page with the required keyword which can help in seo.
keywords are keywords that you shoot with hopes of being in the best position in search engines. while the tag is a word that will be used to support your post with notes, tags will be indexed by search engines
For Search engines, pretty much no difference: it's just that keywords are in the content, title and meta tags, while tags are additional "keywords" that help robots (and people) identify the content in an image, video, audio or Flash website...