Kyrios, the Greek for Lord, appears about 740 times in the New Testament, and usually refers to Jesus.
The use of kyrios in the New Testament has been the subject of debate among modern scholars, and three schools of thought exist on that topic.
The first is that based on the Septuagint usage (the Greek translation of the Old Testament Scriptures which came out 200 years before Jesus was born), the designation is intended to assign to Jesus the Old Testament attributes of God. The reasoning here being, that at the time the Septuagint was written, when reading out loud, Jews pronounced Adonai, the Hebrew word for "Lord", when they encountered the name of God, "YHWH", which was thus translated into Greek in each instance as kyrios. And the early Christians, the majority of whom were speakers of Greek, would have been deeply familiar with the Septuagint.
The second school of thought is that as the early Church expanded, Hellenistic (Greek) influences resulted in the use of the term.
The third is that it is a translation of the Aramaic title Mari applied to Jesus.
One consequence of the use of kyrios to refer to Jesus in the New Testament is that almost all Old Testament references to God (except God the Father and the Holy Spirit) can then apply to Jesus. Kyrios is a key element of the Christology of Apostle Paul. Most scholars agree that the use of kyrios, and hence the Lordship of Jesus, predated the Pauline Epistles, but that Saint Paul expanded and elaborated on that topic More than any other title, kyrios defined the relationship between Jesus and those who believed in him as Christ: Jesus was their Lord and Master who was to be served with all their hearts and who would one day judge their actions throughout their lives.
Cited from http://en.wikipedia.org/wiki/Kyrios with additions made for further clarification