The major use of tenses in the English language is to indicate the "time" frame. Learning tenses may seem difficult to many learners of English, especially foreign learners, because in some languages there is no such a thing as "tenses"-- instead, the "time" factor is indicated by adverbs, such as "tomorrow," "yesterday," "today," or "many years ago."
In the English language, the "time" element is always expressed in different tenses: the Present Tense, the Past Tense, the Present Perfect Tense, the Past Perfect Tense, the Present Continuous Tense, the Past Continuous Tense, the Future Tense, the Future Continuous Tense, and the Future Perfect Tense. In addition, tenses are used to express the "reality" of something, such as the use of subjunctive mood to indicate if something is real or unreal.
THE PRESENT TENSE and THE PRESENT CONTINUOUS TENSE:
The Present Tense is used to express an idea or an event not only true at the present time, but also true all the time or at least most of the time.
e.g. The earth is round. (This is a fact, and it is true all the time -- not just right now. The earth was round when it was created, and it is still round.)
e.g. The sun rises in the east. (Again, this is a fact.)
e.g. Obama is the President of the United States. (Obama may or may not be the President in 2013, but for now he is the President.)
e.g. My sister sings beautifully. (This is the opinion of those who know her that she can sing well.)
The Present Continuous Tense is used to indicate that an action is going on or continuing at the present moment.
e.g. You are reading my blog page on the Present Tense and the Present Continuous Tense.
If you say "my sister is singing beautifully (the use of the Present Continuous Tense)," you are referring to "what she is doing right now -- singing beautifully." But you can also say "my sister sings beautifully" when you are referring not just to "what she is doing right now -- singing beautifully" but also to the fact that "she is always a good singer." See, you can use both the Present Tense and the Present Continuous Tense; it all depends on what you are referring to.
There is another use of the Present Continuous Tense: to indicate an action or event that will definitely take place very soon.
e.g. He is coming home tomorrow (an event that is definite and will happen very soon).
e.g. He will come (the Future Tense) home next week (a mere statement of a future event).
To sum up, you use the Present Tense for what is true or factual all the time, or at least for a certain period of time. Other than that, it is more appropriate to use the Present Continuous Tense for a present event or an action that will happen soon.
To learn more tenses in greater detail, go to Effective Writing Made Simple.