When to use DataContract and DataMember attributes?

I am very confused about the DataContract attribute in WCF. As per my knowledge it is used for serialization user defined type like classes. I write a one class which is expose at client side.

[DataContract]
public class Contact
{
    [DataMember]
    public int Roll { get; set; }

    [DataMember]
    public string Name { get; set; }

    [DataMember]
    public string Address { get; set; }

    [DataMember]
    public int Age { get; set; }
}

It is working properly but when I remove DataContract at class level as well as DataMember it is also work properly. I can't understand that if it is working properly so why there is a need of DataContract? Can any one tell me what is the actual use of DataContract?

My service contract looks like this

[ServiceContract]    
public interface IRestServiceImpl
{
    [OperationContract]        
    Contact XmlData(string id);      
}

解答：

Since a lot of programmers were overwhelmed with the [DataContract] and [DataMember]attributes, with .NET 3.5 SP1, Microsoft made the data contract serializer handle all classes - even without any of those attributes - much like the old XML serializer.

So as of .NET 3.5 SP1, you don't have to add data contract or data member attributes anymore - if you don't then the data contract serializer will serialize all public properties on your class, just like the XML serializer would.

HOWEVER: by not adding those attributes, you lose a lot of useful capabilities:

without [DataContract], you cannot define an XML namespace for your data to live in
without [DataMember], you cannot serialize non-public properties or fields
without [DataMember], you cannot define an order of serialization (Order=) and the DCS will serialize all properties alphabetically
without [DataMember], you cannot define a different name for your property (Name=)
without [DataMember], you cannot define things like IsRequired= or other useful attributes
without [DataMember], you cannot leave out certain public properties - all public properties will be serialized by the DCS

So for a "quick'n'dirty" solution, leaving away the [DataContract] and [DataMember] attributes will work - but it's still a good idea to have them on your data classes - just to be more explicit about what you're doing, and to give yourself access to all those additional features that you don't get without them...