Marker Interface doent contain any methods.These interfaces like SingleThreadModel,EnterpriseBean. These Interfaces are use for some indications.While Running your programme(Servlet),If you want thread safe, the jvm checks are you implement the these interfaces or not.Like this we have several marker interfaces. Like this marker interfaces are using for indication(It Indicates some activity).
Marker interfaces, as per my understanding, are used to indicate your program that the class is expected to show some behaviour (for example, can be serialized if it impliments the serializable interface). Using reflection, you can get the name of the interface that implimented the class. This interface name is then verified to make sure whether the class should impliment certain functionality or not.