The PRESENT PERFECT TENSE is formed with a present tense form of "to have" plus the past participle of the verb (which can be either regular or irregular in form). This tense indicates either that an action was completed (finished or "perfected") at some point in the past or that the action extends to the present:
I have walked two miles already [but I'm still walking].
I have run the Boston Marathon [but that was some time ago].
The critics have praised the film Saving Private Ryan since it came out [and they continue to do so].

The choice between Present Perfect and Simple Past is often determined by the adverbial accompanying the verb. With adverbs referring to a period gone by, we would use the simple past:
I studied all night/yesterday/on Wednesday.
With adverbs beginning in the past and going up to present, we would use the present perfect:
I have studied up to now/lately/already.
An adverbial time-marker such as "today, this month," or "for an hour" can take either the simple past or present perfect:
I worked/have worked hard today.
We tend to use the Present Perfect when reporting or announcing an event of the recent past:
The company's current CEO has lied repeatedly to her employees.
But we tend to use the Simple Past when reporting or announcing events of the finished, more distant past:
Washington encouraged his troops.
Because the time limits for Present Perfect are relatively elastic (stretching up to the present), it is somewhat less definite than the Simple Past:
Brett has worked with some of the best chefs of Europe [in the course of his long and continuing career].
Brett worked with Chef Pierre LeGout [when he lived in Paris].
(Notice how the topic of Brett's work is narrowed down as we move from Present Perfect to Simple Past.)

Authority for the last two paragraphs: A University Grammar of English by Randolph Quirk and Sidney Greenbaum. Longman Group: Essex, England. 1993. Used with permission. Examples our own.