Java class loaders are used to load classes at runtime. ClassLoader in Java works on three principles: delegation, visibility, and uniqueness. Delegation principle forward request of class loading to parent class loader and only loads the class if the parent is not able to find or load the class. The visibility principle allows the child class loader to see all the classes loaded by the parent ClassLoader, but the parent class loader can not see classes loaded by a child. The uniqueness principle allows one to load a class exactly once, which is basically achieved by delegation and ensures that child ClassLoader doesn't reload the class already loaded by a parent. Correct understanding of class loader is a must to resolve issues like NoClassDefFoundError in Java and java.lang.ClassNotFoundException, which is related to class loading.