http://en.wikipedia.org/wiki/Present_perfect
The present perfect tense is a perfect tense used to express action that has been completed with respect to the present. (The word perfect in its name refers to the idea of completion—of being now finished—rather than to perfection in the sense of "no flaws".) "I have finished" is an example of the present perfect. The present perfect is a compound tense in English (and in many other languages), meaning that it is formed by combining an auxiliary verb with the main verb. For example, in modern English, it is formed by combining a present-tense form of the auxiliary verb "to have" with the past participle of the main verb. In the above example, "have" is the auxiliary verb, whereas the past participle "finished" is the main verb. The two verbs are sometimes labeled "V1" and "V2" in grammar instruction.
Present perfect from
Collins Cobuild English Dictionary
http://diccionario.reverso.net/ingles-cobuild/present%20perfect
In grammar, the present perfect tenses of a verb are the ones used to talk about things which happened before the time you are speaking or writing but are relevant to the present situation, or things that began in the past and are still happening. The simple present perfect tense uses `have' or `has' and the past participle of the verb, as in `They have decided what to do'.
SEE ALSO
http://www.languageguide.org/english/grammar/esp/part3/present.jsp
http://ingles.blog.linkua.com/2008/12/12/uso-del-presente-perfecto-en-ingles-use-of-the-present-perfect/
http://www2.college-em.qc.ca/prof/epritchard/preperfq.html
http://www.better-english.com/easier/forsince.htm
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