Hei hei,
one fjord is a narrow, often and sometimes long (up to 200 km) part of the sea;
mostly named in Norway, but also in NewZealand, Alaska or Antartica;
the classical fjord might be deeper than the sea itself;
the fjords exist since the last ice-time, mountain-parts got higher, but heavy ice-masses pressed some mountain-parts below forming long and narrow valleys; at a certain time, in the west of South-Norway, the long lakes "get contact with the sea", fresh and salt water was mixed up.
puh, hopefully, you understand.
If a "fjord" is surrounded by flat countryside, for example the Swedish City Stockholm, you called it there (in German) "Schären"
Hopefully it was a help for you
Med hilsen (Norwegian)
Auf Wiederlesen
Andree