To drive his point home that the Corinthians are acting immaturely through their divisiveness, Paul introduces a couple of metaphors to illustrate the proper place of human ministry - and God's.
The first analogy Paul draws for these baby Christians is that of a field. Beginning in verse 6, Paul explains that his calling was to "plant," meaning to preach the gospel where it had never been heard before. After Paul's introduction of Christ, Apollos' job was to come along and "water" or otherwise nurture the budding faith of the Corinthians and cultivate their spiritual growth. Rather than being in competition with one another, Paul and Apollos each has his own purpose toward the same end. Besides all this, it is neither the planter nor the waterer who is really responsible for the end result. That's up to God.
Paul then switches metaphors in verse 9 ("You are God's field; God's building.") The word picture now is one of construction. Paul's ministry was to lay the foundation of Christ at Corinth. Apollos came later and did some of the building on that foundation. Again, each has his own set of gifts and graces for ministry, but those who build must take care how they do so. The quality of their craftsmanship will one day be revealed and judged, so everyone should do their best.
Incidentally, Paul continues the metaphor into vs. 16 and 17. What kind of building is this that is under construction at Corinth? It is none other than God's temple - a home for His Spirit. Picture that the next time you think of the Church of Jesus Christ!