The cross was an instrument of torture used to execute criminals publically and shamefully in the ancient Roman Empire. The New Testament states that Jesus was executed in this fashion. For this reason, the cross has become the symbol of Christianity.
Symbolism
In pre-Christian times the cross was widely used as a symbol in various places and times. Scholars attribute this wide usage to a psychologicl appeal the shape has; it's proportions are roughly like a human's.
In later pre-Christian times the cross was a symbol associated with phallic worship and sex rituals.
After the spread of the Christian church the usage of cruxiform imagery became wildly popular. Cross shapes are used to decorate most churches, as well as the homes and jewelry of Christians or those influenced by Christian culture.
Idoloatry?
The frequent use of the cross in conjunction with prayer and religious services have caused some Christians to wonder whether or not this practice is idolatrous. However many Christians feel comfortable using the cross because the image itself is not the one on the "receiving end" of the prayer, but rather the person it symbolizes.
True Cross
The cross on which Jesus was crucified became known as the True Cross. A number of medieval legends and relics are centered on the true cross.
The Shape of Christ's Cross
Since the Romans used a variety of cross shapes when inflicting crucifixion and the gospels do not give a description, the shape of the cross Christ died on has been the subject of debate. The greek word, "Stauros", is often translated into "stake" in English. For this reason some groups (like the Jehovah's Witnesses) believe that the cross consisted of a single vertical element and lacked the crosspiece that gives a traditional cross its distinctive shape. Some Bible translations use "tree", when translating "Stauros," which gives credence to the idea.
Another common depiction of the cross is the tau cross, so called because it is shaped like the Greek letter tau, or the english "T." This was a common shape used for executions and differs from the traditional cross in that it lacks the vertical extension above the crosspiece.
A less common form of crucifixion took place on the X shaped "St. Anthony's Cross."
One of the few descriptive details of the cross in the Bible is that there was a plaque above Christ's head. This makes a tau cross or St. Anthony's cross unlikely and further supports the "stake" and traditional cross views.