An appraisal is a professional estimate of a property's market value, based on recent sales of comparable properties, location, square footage and construction quality. This service varies in cost depending on the price of the home. 

A comparative market analysis is an informal estimate of market value performed by a real estate agent based on similar sales and property attributes, usually found in an MLS system. 

Most agents offer free analyses in the hopes of winning your business. You also can get a comparable sales report for a fee from private companies that specialize in real estate data or find comparable sales information available on various real estate Internet sites. 

A so-called comparative market analysis provides the background data upon which to base your list-price decision. When you prepare to sell and are interviewing agents, study each agent's comparable sales report (the data should be no more than three months old). If all agents agree on a price range for your home, go with the consensus. Watch out for an agent whose opinion of value is considerably higher than the others.