A security may be listed in more than one market. It is common for one such listing to be a primary listing and the others secondary listings.
Having multiple listings gives an issuer access to a wider pool of money. Although there are mechanisms that allow multiple primary listings these are more complex and expensive.
Liquidity tends to shift to the market where the primary listing is. Companies also tend to be less committed to the markets in which they have secondary listings: an example of this is the speed with which British companies got rid of the secondary listings in the US when the Sarbanes-Oxley legislation made US listings more expensive.