Companies and therefore their stocks are broken up into different market sectors, with companies sharing similar characteristics such as Financial companies being grouped together into one sector. As a trader you will often hear that a particular sector is weak or strong which generally means that the stocks which make up that sector have been trading lower or higher over the time period referenced. Many traders who use a top down style of analysis will reduce the number of stocks they are watching by looking at only the strongest sectors, and then trading stocks within those sectors.