Lookup filter is used to filter records when we lookup for the records.
Used in Lookup dialogs.
http://infallibletechie.blogspot.com/2013/04/lookup-filter-in-salesforce.html
2)When a task is assigned to user through web to lead or web to case through assignment rules,where the tasks are appeared for that particular user?
Home page of the user.
http://infallibletechie.blogspot.com/2013/04/where-to-view-assigned-tasks-in.html
3)When a case is generated by an user through web to case,how or where a developer will provide solution case arised?
Email notification through trigger or through email alert Workflow rule.
4)what is the use of interfaces(in apex classes)?
An interface is like a class in which none of the methods have been implemented—the method signatures are there, but the body of each method is empty. To use an interface, another class must implement it by providing a body for all of the methods contained in the interface.
Interfaces can provide a layer of abstraction to your code. They separate the specific implementation of a method from the declaration for that method. This way you can have different implementations of a method based on your specific application.
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_classes_interfaces.htm
5)I have added an string ‘updated’ to all users in Account object through batch apex,now how to remove that ‘updated’?
Run the below code in developer console
List<Account> acc =[SELECT Id, Name FROM Account];
for(Account a : acc)
{
a.Name = a.Name.removeEnd(‘Updated’);
update a;
}