Iceberg: definition.
Icebergs are thick bits of ice that break off from the glacial ice that flows from the Antarctic continent and continents surrounding the Arctic ocean into the sea. Icebergs are made of fresh water, representing hundreds of thousands of years of accumulated snow.
In contrast, sea ice is much thinner, often thin enough to be cut apart by ice breaker ships. Sea ice forms by freezing of the sea surface when the atmosphere is cold and incoming radiation is weak (fall and winter). While sea ice is fresher than the seawater it forms from, it still contains a fair bit of salt