Shrouds are the lines that run from the top or middle of the mast down to the deck on either side of the mast. Shrouds are part of a boat's standing rigging and are usually steel cables that are attached to the deck with a chain plate and tightened with turnbuckles.
A mast may have one or more sets of spreaders, which are horizontal spars that cross the mast like a lowercase "t".
The upper shrouds (or "uppers") are the shrouds that go from the deck all they way to the top of the mast, usually through the tips of the spreaders.
The lower shrouds (or "lowers") are the shrouds that only go partway up the mast.