When describing the hull of a vessel three basic measurements give a rough outline of the shape of the hull. These are Length, Beam, and Draft.
Beam is a measurement of a vessel's width. It is always measured at the widest point because it is often used to determine if passage can be safely made near an obstacle.
Beam is important in determining the handling characteristics of a ship design. A narrow beam hull will run fast but will not perform well in heavy waves because of the narrow cross section. A hull which has a wider beam will be less efficient in cutting through the water because of the larger mass of water that is being displaced. This larger mass also tends to roll less.
Beam can also be measured at specific points on the hull like the pilot house or cargo area but these measurements will be designated with the names of these structures. The main measurement of beam is taken at he widest point of a vessel.
Naval architects use length, beam, and draft measurements to shape a hull for a specific job by using the concept of Deadrise. The three main hull measurements along with deadrise give the hull a specific shape and handling characteristics.
You may hear someone refer to a vessel as "Beamy". This means that a vessel has a wide beam in proportion to her length.