The Present Continuous is used to denote an action in progress going on at the present moment.
We are having a lesson.
У нас урок.
Note: One should bear in mind that the notion the present moment can be regarded wider than the moment of speech.
I am reading a very interesting detective story.
Я читаю очень интересный детектив.
The Present Continuous can be also used to denote continual processes.
The Earth is rotating.
The Present Continuous can be used emphatically instead of the Present Indefinite to denote a customary repeated action in the present when the speaker wants to represent it as a continual process showing his emotional attitude to this action (usually negative).
They are always quarrelling.
Они постоянно ссорятся.
The Present Continuous is often used in adverbial clauses of time introduced by the conjunction while.
Watch TV while I am making coffee.
Посмотри телевизор, пока я делаю кофе.
The Present Continuous can be used to denote a future action that will take place in the near future and is regarded as fixed.
We are going to the country tomorrow.
Мы завтра собираемся загород.