Sectors are the broadest way of grouping public companies. The standard system (called GICS) has 11 of them: Energy, Materials, Industrials, Consumer Discretionary, Consumer Staples, Healthcare, Financials, Information Technology, Communication Services, Utilities, and Real Estate.
Knowing a stock's sector matters for two reasons. First, some numbers (such as return on equity, debt-to-equity, and gross margin) only make sense when you compare companies inside the same sector. Second, sectors behave very differently as the economy goes up and down.