Loch is a Gaelic word for lake. It is commonly used in Scotland. The 'ch' at the end is pronounced with a guttural (back of throat) sound which does not occur in modern English. There are also lochs in Ireland, and the word is sometimes used in other countries.
The word is used both for lakes, and for "narrow or landlocked arms of the sea" (OED). The word comes from Proto-Indo-European *lókus ("lake, pool"). It is related to the Latin lacus ("lake, pond") and English lay ("lake").
Sometimes the term sea loch is used for saltwater lochs. Many other lochs are normal freshwater lakes, like Loch Lomond.
The spelling "lough" is often found in place names. In Lowland Scots and Scottish English, the spelling "loch" is always used.