The present perfectThe aspect expressing completion. E.g.: He’s finished university. tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses. is used to express actionA general term expressing that somebody or something does somethings happening in an incomplete period of time. An incomplete period is one which includes the present moment, that is, the time of speakingA point of time at which an utterance is made.. For example, the time expressionA word or group of words used in a particular situation. E.g.: Good morning, at six, on the sofas today, this week, this year, in my life etc. all refer to such a period.

I haven't smoked today.
I've lived here all my life.
Have you ever been to China? (the implied incomplete period here is "your life")
My brother has written several plays. (in his life, which is incomplete as he is still alive)
I have never driven such a fast car.

If we mean a completed period, which does not contain the time of speaking, we use the past simple tense instead of the present perfect.

I lived in this house when I was a child. (I am not a child any more.)
Did you go to China when you worked as a flight attendant?
Shakespeare wrote several plays. (in his life, which is completed as he is dead now)
He worked in this café between 1905 and 1906.
I didn't smoke yesterday.

Note that some time expressions (e.g. this morning, recently, lately) can refer to either a completed or an incomplete period depending on the contextThe situation within which something exists or happens, or an utterance is made. and whether the time of speaking is included or not in the given period. Compare these two sentences:

I have seen him this morning.
I saw him this morning.

The first sentence suggests that it is still "this morning", so the time of speaking is before approximately 1 p.m. The second sentence is said later during the day.  

Here is another example:

He has put on a lot of weight recently.
He gained some weight recently.

In the first sentence recently means in the past few days/weeks/months etc, which period includes the time of speaking, while in the second sentence it expresses some time ago, which refers to a point of timeA particular moment in time. E.g.: She came home at six. ("six") I was born in 1980. ("1980") in the past.

Note that we do not always use the present perfect tense even if we mean an incomplete period of time. Compare these two sentences:

Have you been to Lake Balaton this year?
Did you go to Lake Balaton this year?

In the first sentence we are merely curious if our friend has visited the place any time during this year (indefiniteNot relating to a specific or determined person, thing, idea or time. E.g.: Somebody left you a message. ("somebody", "a message") time, incomplete period) so we use the present perfect tense. In the second case, however, we know that he goes to Lake Balaton every year, usually around the same time. What we ask is whether he went there this year (at that time) as well. Here we mean a definite time in an incomplete period so we use the past simple tense.

Diagrams

Present perfect for recent events with a result in the present
Present perfect Present perfect with superlative forms of adjectives

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