Prophecy in a broad sense, is the prediction of future events or the speaking of divine words (divine Revelation) through chosen human messengers (prophets). The etymology of the word is Greek, from pro- "before" plus the root of phanai "speak", i.e. "speaking before" or "foretelling" (proclamation). Prophecy is a general term for assertions presented as the revelation of divine will. Sometimes the Greek word mantikę (divination) is translated as prophecy.
In the Bible prophecy is often referred as one of the fivefold ministries or spiritual gifts that accompany the indwelling of the Holy Spirit. The five ministries being; Apostles; Prophets; Evangelists; Teachers and Pastors. (Eph. 4:11) The focus of prophecy is not just future events though, this is only part of the prophetic gifting. Prophets often brought words of comfort, exhortation or general upliftment to the Church. Paul teaches in Corinthians that it is for the benefit of the whole body. It is important for God to speak to believers as he does through his prophets.
* A working definition of the gift of prophecy
Note that gifts vary from one believer to another, and also vary in the way they
outwork (Rom 12:6, 1 Cor 12:4-11).
"The gift of prophecy is a grace bestowed upon a believer so chosen by the Lord to bring an utterance of divine nature. This utterance is for the strengthening, encouragement and comfort of the Body of Christ. He who prophesies builds up the church. The word so spoken may be future telling, forth-telling, expounding, a word of knowledge, a word of wisdom, some instruction, direction, a song, poem or other communication"
* A model of understanding the "gift of prophecy" and the "Office of Prophet"
Mike Bickle, senior pastor of Metro Christian Fellowship of Kansas City, offers
a very helpful model to understand the continuum from simple (ordinary) prophecy to the office of prophet:
I Simple prophecy
II prophetic giftedness
III prophetic ministry
IV Prophetic Office
I) Simple prophecy - any believer speaking something God has brought to mind.
II) Prophetic giftedness - believers who receive dreams, visions, impressions and share such revelation without necessarily having sharp clarity in
understanding it all. There are potentially a lot of people in this category.
III) Prophetic ministry - believers who have been recognised and commissioned for regular ministry in the local church setting. Revelation is discerned, interpreted and applied. There are much fewer in this position.
IV) Prophetic Office - Believers whose ministry is followed by signs and
wonders, are judged to speak accurate words from God and whose credibility has been clearly established by their track record. There are very few people in this category.
The Gift
Manifestations
There are three Hebrew words used in the Old Testament to describe the ways in which the gift of prophecy may come forth in the life of a believer. It pays to understand the workings of this gift, to assist in ensuring its correct
deliverance.
NABA: (naw - baw) meaning 'to bubble forth'. This form of prophetic utterance
springs forth spontaneously by inspiration. It is therefore critical to allow it
to manifest spontaneously. Such words often come during worship, prayer and intercession times.
NATAPH (naw - taf) meaning 'to distill gradually'. This is a forming, developing of the word, like ruminating. This form of prophetic utterance is slow to
develop and comes like the dawn. It is possible to write such inspiration down
and deliver it.
CHAZAH (khaw - zaw) meaning 'to perceive' or envision. This literally means to see or have a vision, or discern something in the spirit realm. Such visions are also delivered verbally or in written form.
Boundaries for the gift
Simple prophecy, and the operation of prophetic gifts per se should not be
dangerous. It's sole function is the edification, building up and exhortation of the Body. Paul states this as being the boundary for the operation of the word of prophecy in 1 Corinthians 14:3:
"Everyone who prophesies speaks to men for their strengthening, encouragement and comfort... he who prophesies builds up the church"
Prophesy delivered by believers to the Body should eliminate confusion, not
bring further doubt and trouble (1 Cor 14:33). Note that edification,
exhortation and comfort can still be somewhat negative or challenging - just not destructive.
Testing prophecies
Because prophetic words come through sinful, human vessels it is important that we test all words (1 Thess 5:19-21). Paul says that we should not despise prophetic utterance but rather test all things and hold fast to that which is good. This is why TAG recommends a viable testing, similar to the “3 Legged Stool”*, be used every time a word is given ~ whether you give it or receive it.
(* 3 Legged Stool test is from Brad Jersak, at Fresh Wind Prophetic Community.)
These trained prophetic teams will be available at various times during TAG events. 
Prophesy can be “tested” in many ways. One of these ways is “the three-legged stool”.