They are not that different no. When people refer to "keywords", they usually refer to the words and phrases that people type into search engines to find your hub. These keywords are also used in the content of your hub. The tags are a set of words and phrases that tell us what you hub is about. It is a good idea to include your keywords there but also other words and phrases that you did not end up using.
These tags can still attract people to your hub because for every tag, there is a page that mentions all the hubs with that tag. That is an easy way to get more backlinks from the hubpages.com domain to your hub and it helps to associate your hub with those words and phrases.