Keyword - a word that people will text to your shortcode to join your List. You can have multiple Keywords for different ministries, events, sign ups, etc. You are not limited to using just one Keyword at a time. Example: Text Connect to 970-00. You can send a series of auto-responses to people that text in with your Keyword.
Subscriber Lists - are where your contacts are stored. Each Keyword you create can tie to one or more Subscriber Lists. Each account can have an unlimited number of Lists. People can join a list through a Keyword, but they can also join through other ways.
Note: if you delete a Keyword, you will not lose the list that it's tied to. You will still have that list and any contacts in it.