The -ing ending is used to show the progressive
aspect (progressive / continuous verb tenses).
The progressive aspect shows that an action is / was /
has been / had been / will be (etc.) in progress at a
particular time or during a particular period of time.
It often suggests that the action is / was / has been /
had been / will be (etc.) long or uninterrupted.
All of the English tenses can use the progressive aspect--
and in both active and passive sentences. Progressive
aspect always has at least two parts: the verb be (which
is often contracted) and an - ing verb: