Tammy, a "transit" is the motion of a planet in its orbit, and refers particularly to the moment in time when any transiting planet (planet in its real-time motion) crosses over, or comes into astrological aspect to, a point in the zodiac that was occupied by a planet at your birth, that is, in your natal chart.

Suppose the Sun was in the 13th degree of Libra the day you were born. The astrologer would say that "she has the Sun in 13 Libra". This point is "sensitized" and remains so throughout your life. Today, in real-time, Saturn is in 13 Libra. So "Saturn is transiting your Sun".

The transits, especially of the "heavy" planets (those from Jupiter outward to Pluto), "cause" conditions and events to occur in your life. They are one of the primary "predictive tools" used by astrologers.

They can be either "good" or "bad". What is important is that the show the timing of events within rather close limits. It is the natal planet that is "activated", that is "acted upon". The transiting planet, the one in real-time motion affecting the natal planet, is the one that brings events or conditions from "outside" (it can be internal as well) and sets off the "meaning" of the natal planet.

I like morbid examples. They shock people. So I will use the transit of Pluto. Today Pluto is retrograde in the 6th degree of Capricorn. Pluto moves very slowly (his orbital period is about 250 years -- he goes around the Sun one time every 250 years) and so he will not reach 13 Capricorn until spring of 2014. My Ascendant, one of the astrological symbols of our "life", is in 13 Cancer, exactly opposite 13 Cancer. So we say, when Pluto comes to 13 Capricorn, that "Pluto opposes the natal Ascendant". Pluto is the planet of transformation, the Ascendant the "body, the life". Thus I will be transformed in the spring of 2014; I will die. [The fact that Pluto opposes the Ascendant does not always indicate the personal death. Many other factors were considered in forming this prediction.]

When Jupiter transits your Sun (the conjunction) you may win the lottery. Or get married. Or go on a long and self-expanding trip. What actually happens depends on the natal chart. Most transits "cause" several more or less simultaneous events in a life (or destiny of a nation) that seem to be unrelated but are really different manifestations of the same energy.

Hope this sheds some light on your question.