This is a practical oversimplification which will be right most of the time. For more detailed information see Mastery of the Tenses!
For things happening now
- He's coming over!
- She's talking to her now.
- I am reading a book.
For things changing
- The world is getting more populated.
- My English is improving.
For temporary things
- We are only working here this week.
- I am being stupid today!
- She is not eating chocolate this month.
For future personal plans, especially with confirmed time/place
- I am seeing my dentist on Tuesday.
- She is meeting John at 3pm.