Sea ice is simply frozen ocean water. It forms, grows, and melts in the ocean. Unlike sea ice, icebergs, glaciers, ice sheets, and ice shelves all originate on land. Sea ice grows in the Arctic and Antarctic. It grows during the winter months and melst during summer months, but some ice remains all year in certain regions. About 15% of the world's oceans are covered by sea ice during part of the year.

Even though sea ice occurs primarily in the polar regions, it influences our global climate. Sea ice has a bright surface, so most of the sunlight that strikes it, is reflected back into space. This means areas covered by sea ice don't absorb much solar energy, so temperatures in the polar regions remain relatively cool. If gradually warming temperatures melt sea ice over time, fewer bright surfaces are available to reflect sunlight back into space, more solar energy is absorbed at the surface, and temperatures rise further. This starts a cycle of warming and melting. This cycle is temporarily halted when the dark days of the polar winter return, but it starts again in the following spring. An early small temperature increase leads to greater warming over time, which makes the polar regions the most sensitive to climate change on Earth.

Sea ice also affects the movement of ocean waters. When sea ice forms, some of the salt in the ocean water is trapped in small pockets between the ice crystals; however, most of it is "ejected" into the ocean below. Because salty water is heavier, the density of the water increases and the water sinks. This “overturning” of the ocean waters contributes to the earth's overall ocean circulation: cold, polar water moves along the bottom of the ocean toward the equator and warm water from the equator travels along the ocean surface toward the poles. This circulation is needed to maintain mild temperatures in places like Europe. So, changes in the amount of sea ice ultimately disrupts the normal ocean circulation.

Sea ice can also be an obstacle to normal shipping routes through the Northern Sea route and Northwest Passage. Finally, wildlife and people rely on sea ice to hunt and travel.

Sea ice grows, forms, and melts strictly in the ocean. Glaciers are considered land ice. Icebergs are chunks of ice that break off from glaciers and dump into the ocean. Lake ice is made from fresh water and freezes as a smooth layer, unlike sea ice, which develops into various forms and shapes because the ocean water is constantly mixing. Sea ice forms differently from lake or river ice because it forms from salty ocean water instead of fresh water.

Fresh water is unlike most substances because it becomes less dense as it nears the freezing point. This explains why ice cubes float in a glass of water. Fresh surface water near the freezing point stays at the surface, so river and lake ice form at the surface.

However, in contrast to fresh water, the salt in ocean water causes the density of the water to increase as it nears the freezing point, and ocean water tends to sink. So, sea ice forms slowly, compared to freshwater ice, because salt water sinks away from the cold surface before it cools enough to freeze. Ocean water is very deep and requires more time to reach the freezing point. Also, the freezing temperature of salt water is lower than fresh water: the ocean typically freezes at -1.8 degrees Celsius. The top 100 to 150 meters of water must be cooled to this temperature for ice to form. In the Canadian Basin of the Arctic Ocean, the constantly mixing top layer of water is only 50 to 60 meters in the winter.