I want to know the major differences between these two systems.
Geologically significantly different. Fjords are flooded glacial valleys, and usually have very steep sides. Often the fjord itself is deeper than the ocean beyond it's mouth. Many have very inaccessible shorelines except at the extreme ends.
Loch is used both for lake, and for a confined or protected sea inlet.
Firth is another term, tends to refer to a more open body of water. The use of the terms is not consistent.