progressive 1: general

forms
Progressive verb forms (also called ‘continuous’ forms) are made with be + -ing.
I am waiting for the shops to open, (present progressive)
Your suit is being cleaned,
(present progressive passive)
She phoned while I was cooking, (past progressive)
I didn’t know how long she had been sitting there, (past perfect progressive) Will you be going out this evening? (future progressive)
I’d like to be lying on the beach now. (progressive infinitive)
terminology and use
A progressive form does not simply show the time of an event. It also shows how the speaker sees the event – generally as ongoing and temporary, not completed or permanent. (Because of this, grammars often talk about ‘progressive aspect’ rather than ‘progressive tenses’.) Compare:
I’ve read your letter, (completed action)
I’ve been reading a lot of thrillers recently, (not necessarily completed)
The Rhine runs into the North Sea. (permanent)
We’ll have to phone the plumber – water’s running down the kitchen wall. (temporary)
When a progressive is used to refer to a short momentary action, it often suggests repetition.
Why are you jumping up and down?
The door was banging in the wind.
distancing
Progressive forms can make requests, questions and statements less direct. (They sound less definite than simple forms, because they suggest something temporary and incomplete.)
I’m hoping you can lend me Ј 10. (less definite than I hope …)
What time are you planning to arrive?
I’m looking forward to seeing you again.
I’m afraid we must be going.
I was wondering if you had. two single rooms.
Will you be going away at the weekend?
For more about this kind of ‘distancing’, see politeness 2: distancing verb forms.
For more details of the use of progressives, see the individual entries on the present progressive, past progressive etc.