When a glacier flows into the ocean it will start to form icebergs and you say that the glacier is calving. Icebergs are formed when bits of the glacier breaks off and fall into the water.
Watch this little animation video showing what happens when a glacier is calving large icebergs. In the next video, you can see how this looks in real life.
In this video you can see what it looks like when a real glacier is calving. Helheim is an outlet glacier in South-East Greenland and it is one of the fastest flowing glaciers in Greenland. In this video you can see a very large iceberg breaking off Helheimgletscher (glacier).
This animation video shows how meltwater can help calving. Water from the surface of the glacier is transported to the base of the glacier, where it runs out to the fjord. When water runs under the glacier, it will start sliding over the bed and thereby move faster out into the fjord where it breaks up into icebergs.