A class loader is a part of the java run time environment that dynamically loads java classes into the java virtual machine. It is basically a sub-system of JVM which is used to load class files . The class loader is responsible for locating libraries, reading their contents,and loading the classes contain within the libraries.
This loading is typically done *on demand*, in that it does not occur untill the class is called by the program. The java run time system does not need to know about files and file system as this are delegated to the class loader .when the JVM is started ,three class loaders are used: bootstrap class loader, extension class loader, system class loader.
