This well known series indicates the relative nobility of different metals and alloys in seawater, based on the measurement of corrosion potentials. In a galvanic cell, the more noble material in this series will become the cathode (no metal dissolution), while the less noble material will corrode as the anode. A greater separation of the materials in the galvanic series indicates a bigger potential difference between the materials; generally indicating a greater degree of galvanic incompatibility when coupled. 

Note that this galvanic series was derived for one specific electrolyte (seawater) only. The materials can have a different nobility ranking in different environments and at different temperatures. The series is based on (averaged) corrosion potential data and therefore does not give a direct indication of the rate of galvanic corrosion.  Selected metals/alloys are shown below in a simplified version of this series:

The position of individual alloys within an alloy family depends on their more specific chemical composition. For example, more detailed series have been published to show the relative positions of different aluminum alloys, bronzes and austenitic stainless steels. The difference in position of certain alloys, depending on their active or passive surface conditions, should be noted.