Fjords are formed by valley glaciers flowing down from their area of origin, through pre-existing river valleys. The original shape of the valley is overstated by the glacier, in that the ice pulls rock along with it and this further erodes the surrounding rock. The original valley, for example a notched valley, becomes wider and deeper and gets its typical U-shaped valley, also known as a trough valley, with very steep slopes. With the retreat of the glaciers, the sea is able to flow into the deep valleys.

The depth of fjords can vary. In some cases, they may be no more than 25 m (82 ft) deep, while in others, the bottom can be more than 1,000 m (3,300 ft) below sea level. Typical for glacial fjords are relatively shallow thresholds at the mouth creating a relatively closed off basin within. Sognefjorden main basin is more than 1000 m deep while the threshold is some 100 m (330 ft) below the surface; there are thresholds at less than 2 m (6.6 ft) while the fjord within can be 100 or 200 m (330 or 660 ft). There can be strong tidal currents at shallow and narrow thresholds.

Sognefjorden is the second longest in the world and the largest glacier-free fjord complex. Unlike Greenland and Svalbard, mainland Norway's fjords does not have icebergs but inner branches may freeze over in cold winters.

In most places, the spelling of fjord is fjord except in New Zealand where the spelling fiord is the standard and is reflected in place names such as Fiordland National Park. In Canada, fiord is only used in proper names such as Grise Fiord, but it's fjord otherwise. The term is of Old Norse origin and adopted into English from Norwegian. The Icelandic cognate of the word is "fjörður". The original Old Norse meaning is related to crossing or travel as the word ford in English. An older term was angr and survived only as a suffix in names of some Scandinavian fjords and has in same cases also been transferred to adjacent settlements or surrounding areas for instance Hardanger, Stavanger and Geiranger. In many parts of Norway, fjord are not merely landscape features but, together with adjacent tangle of valleys, lakes, glaciers and islands, make up a particular type of landscape. The cognate for "fjord" in the Scots language is firth, though many of Scotland's fjords are simply known as "sea lochs", and not all firths are fjords in the geological sense.

Several Norwegian freshwater lakes that have formed in long glacially carved valleys and are frequently named fjords. Such "fjord-lakes" are very similar to saltwater fjords, and typically appear as extensions of the fjord. Some lake surfaces are barely above sea level while most of the lake can be several hundred meters below sea level. Hornindalsvatnet at more than 500 m (1,600 ft) is the deepest lake in Europe and most of the lake's volume is in fact below sea level. Many lowland lakes emerged from the saltwater during the post-glacial rebound. In a few rare cases saltwater has been trapped for several thousand years at the bottom of some deep lakes, known as meromictic lakes.

Fjords, and associated belt of islands, typically create a very fragmented coastline. Norway's coastline for instance is about 2,500 km measured along the shortest route but more than 25,000 km if fjords are included and 100,000 km if islands are also included. Only Canada has a longer coastline.

• Ice age traces
• Mountain ranges
• Natural attractions