Alaskan cruises generally come in three versions as you noted.
Northbound: this is a cruise that starts in Seattle or Vancouver (in virtually all cases) and ends at a port in Alaska such as Seward or Anchorage.
Southbound: Just the opposite of above.
Roundtrip: Leaves and returns to the same port. This it usually out of Seattle or Vancouver as well, and these go to the Inside Passage.
Inside Passage: The area of the long thin peninsula of Alaska that runs along the coast, separating Canada from the ocean in many places. It is the closest part of Alaska to the 'main' United States. Some say it isn't 'really' Alaska since it isn't in the mainland of Alaska, but I disagree. It is a stunningly beautiful and peaceful area.
Some things to consider: a roundtrip out of Seattle is a closed-loop cruise which relaxes the proof-of-citizenship requirements. A certified birth certificate and photo ID will suffice. Doing a north or south bound is a one-way voyage, and even if it starts in Seattle and ends in Anchorage, U.S. law requires the guests to have a passport or passport card. This is because you have to stop at a foreign port (a Canadian port of convenience in most cases, such as Victoria) and the closed-loop legal exemption is only for cruises that start and end at the same U.S. port.
Now, with Mom being 80 I would suggest you look at the round-trip cruises out of Seattle. You are less likely to encounter rough seas (which do happen on the north or south bound cruises occasionally), and the inside passage is usually like riding on a lake for all 7 days.
Princess is always a great choice for Alaska.