The use case diagram shows the way your system will be use, who will use it (who could be other computers, ...). It also shows inputs and outputs from your system.

Lets talk about the classical login use case diagram... (make yours)

The sequence diagram will show one (an instance thought) use from the general use case ... lets say for exemple a login sequence that will run ok ; or a sequence that will, on the other hand, not run ok. Those are two instances of the same use case (login) but will present different results to the user.

And now you understand that has use case talk about generality, it deals with classes but sequence diagram cannot deal with classes because it's like a real application of your use case. And thus has you show this real application you must deal with instanciated classes (that is : objects)

In the same order of idea, the association shows that to classes are connected or could be, while the link is the computed and instanciated association. One is general way of use, the other one is the use of it.

association : to go from there to there by train, you need a tiket

link : I'm gooing from Berlin to Paris by train tomorow morning and I have got my tiket

Regards