How would you explain the difference between "Keywords" & "tags"?

Keywords are for search engines, tags are for humans

Found a topic about tags and keywords, thought that I am going to share this:

The most important detail to keep in mind is that they're entirely different things, regardless of their outward similarity. Tags are well, tags as in the organizational method like you see on del.icio.us and other bookmarking services. Keywords are just extra words attached to a given entry as meta-data, and those words are included in regular text searches, even though those words may not be displayed in the template output. (So, for example, if you write an entry on tigers, you can include "big cats" in the keywords, and it'll turn up for that even if you don't use that phrase in the entry body.) I'm not absolutely sure of this(checking), but I don't believe that tags are treated in this way.

More practically, the Keywords field never seemed to be very widely used, and most people did and still do just use it as an extra text field.

Regarding entry, see below about tags(which also offer auto-complete). Keywords is just a plain text field whose content is not tracked, as any given set of words only applies to its specific entry. There's no formatting suggestion, either. Input however you like.

From: Difference Between Tags And Keywords? - Movable Type Community Forum