Here (following) is basically a restatement of what others have said, but in a way that I myself would understand better:
When someone goes to a computer to look up information about a topic on the Internet, they type words and phrases into search boxes (such as on the Google, Yahoo, MSN, or AOL home page, for example). Those words and phrases - the search terms - are "keywords," whether they are single words or phrases composed of several words. So these are the terms that Google or whoever will use to find the millions of articles that pertain to that term.
Our goal as writers is to use excellent keywords in our Hub's title, its URL, and in the first paragraph or two of the Hub. Doing so will make our Hub more visible to the search engine. Several people here have written Hubs about how to find the best keywords. You should read and heed their advice, because it's quite probable that someone else somewhere in the world has already written another article on the same topic on another site, using the very same keywords as you.
Different keywords also have different monetary value - and this is getting more complicated than you need here.
Tags are, as the others have said, words and phrases that help within HubPages to identify the topic of a Hub and to link it to other Hubs with similar content. Tags can be keywords, but they also may not be.