Use case question - what is the difference between include and extend?

Question

if I make a book selling system in which users should search or browse book first before buying, should I include/extend the buy use case within the search book and browse book, or should it be done the opposite way?

Answer 1

In system if some use case is dependent on some another usecase then we use include relation ship

e.g. login---> includes---> validation

and when we want some extra facility to be provided by the system apart form original fuctionality then it will need extend relation ship

e.g. order--->extends---> shipping order