Sea ice forms in many varieties in the polar Oceans. The most common type of sea ice, especially in the Arctic Ocean, is "Congelation ice" a solid form of sea ice where new ice congeals onto the bottom of existing ice.
In Antarctic waters (image) much of the sea ice forms under influence of waves, and pancake ice is formed. The pancakes form due to the waves bouncing the small floes against each other during sea ice formation. With time the pancakes grow thicker and larger, and the waves dampen out. Another form of sea ice is the initial loose crystals termed frazil ice. This is individual crystals suspended in water, and they stay suspended below the surface due to vertical mixing.
Sea ice is also moving forced by the wind and currents below. Back in the Arctic Ocean especially, but also in Antarctic waters, this sea ice drift can deform and crush one floe up against other floes to form pressure ridges. Also rafted sea ice can form this way, when one floe is pressed ontop of another. In the Arctic area between the North Pole and Canada the sea ice is largely deformed and has become very thick (>5 m) due to such processes.
BIAC - Bipolar Atlantic Thermohaline Circulation
NorClim - Climate of Norway and the Arctic in the 21st Century
ProClim - Polar Ocean Climate Processes