A fjord is a valley carved out by a glacier that has later been invaded by the sea, leaving salty water. They are usually narrow and bordered by steep mountains, which rise below sea level.
They are found in places where glaciation (present or past) has reached sea level (current). They are formed when a glacier reaches the sea and melts. This leaves a valley in its wake, which is flooded by the sea as the ice retreats. They are usually long, narrow and deep.
The fjords of Flam, Alesund, Stavanger, Hellesylt, Geiranger, Vik, Trondheim, Andalsnes and Molde (Romsdalsfjord) and Oslo (Vikenfjord) are especially well known.
Without a doubt, the most comfortable way to explore the Norwegian fjords is on board one of the spectacular cruises that transport thousands of passengers from all over the world every year who come to Norway to admire the unique beauty of these idyllic natural monuments.
Thanks for the description, it is very illustrative.