The term sports memorabilia usually refers to anything that can be directly connected to a sports event or personality. These items are generally gathered by the fans of the particular sport, athlete or team that the item signifies or by collectors who find value in the rarity time-value of these collectibles. "Sports memorabilia" often refers to autographed sports items as well.
Sports memorabilia can either have a sentimental or monetary value to the collector. Many items used by famous sports stars or at a famous event have been sold for many dollars at auctions such as Sotheby's and others. The ball that Mark McGwire hit for home run number 62 in 1998, for example, was sold for over 1 million dollars.
Adding an autograph to an item used in a particular game or by a particular star can increase or decrease the value of the item, depending on the situation. If the autograph has been personalized (i.e. "To Mark") the autograph devalues the item on the public market. Also, collectors put little value on autographed memorabilia that is sold without a certificate of authenticity. Several companies have developed systems to prove authenticity of game-used or autographed memorabilia, and collectors will generally seek out dealers that offer a lifetime, money-back, authenticity guarantee.
For the collectors who only see their memorabilia as having a sentimental value, they might look at it as a childhood memory, or as an important moment of life shared with his or her favorite star or team.