to describe an action that is going on at this moment: You are using the Internet. You are studying English grammar.
to describe an action that is going on during this period of time or a trend: Are you still working for the same company? More and more people are becoming vegetarian.
to describe an action or event in the future, which has already been planned or prepared: We're going on holiday tomorrow. I'm meeting my boyfriend tonight. Are they visiting you next winter?
to describe a temporary event or situation: He usually plays the drums, but he's playing bass guitar tonight. The weather forecast was good, but it's raining at the moment.
with "always, forever, constantly", to describe and emphasise a continuing series of repeated actions: Harry and Sally are always arguing! You're constantly complaining about your mother-in-law!