From Easton's Online Bible Dictionary...
Kingdom of God
(Matthew 6:33; Mark 1:14, 15; Luke 4:43) = "kingdom of Christ" (Matthew 13:41; 20:21) = "kingdom of Christ and of God" (Ephesians 5:5) = "kingdom of David" (Mark 11:10) = "the kingdom" (Matthew 8:12; 13:19) = "kingdom of heaven" (Matthew 3:2; 4:17; 13:41), all denote the same thing under different aspects, viz.: (1) Christ's mediatorial authority, or his rule on the earth; (2) the blessings and advantages of all kinds that flow from this rule; (3) the subjects of this kingdom taken collectively, or the Church.
Derived probably from the Greek kuriakon (i.e., "the Lord's house"), which was used by ancient authors for the place of worship.
In the New Testament it is the translation of the Greek word ecclesia, which is synonymous with the Hebrew kahal of the Old Testament, both words meaning simply an assembly, the character of which can only be known from the connection in which the word is found. There is no clear instance of its being used for a place of meeting or of worship, although in post-apostolic times it early received this meaning. Nor is this word ever used to denote the inhabitants of a country united in the same profession, as when we say the "Church of England," the "Church of Scotland," etc.
Right now over at Jesus Creed, McKnight has an excellent series on the topic of the Kingdom. You might want to check it out.