The Shamrock is a three-leaf clover. A symbol of Irland, its name derives from from the gaelic word seamrog. This is not to be confused with the four-leaf clover which is a symbol of good luck to the Irish, while the three-leaf clover is more of a Christian symbol denoting the trinity.
In Ireland, the white clover is the plant most noted as the shamrock. Legend has it that St. Patrick used the shamrock as a spiritual teaching tool to explain the Trinity. Thus, the shamrock had become associated with St. Patrick. Knowing the love of the shamrock which the Irish held, St. Patrick was able to attract a large multitude of Celtic pagans into Christianity. However, the symbolism of the Trinity attached to the shamrock probably didn't originate with St. Patrick. The Ancient Celts had a high regard for the three-leaf clover because of the triad of the leaves.
At times the shamrock grows a fourth leaf, known as the four-leaf clover. Since it is quite rare for a fourth leaf to grow on a shamrock, many believe the finder of a shamrock with a fourth leaf will have good luck. the four-leaf clover brings good luck.