Second semester seminary students did a mini-word study on one of the following Greek terms. The following is my thumbnail. And, no, Jeff, you can't turn this in for the onsite class :-)
1. apostle (apostolos)
It seems to me that the word is used in at least three senses in the New Testament: a) of the Twelve--these are the apostles, b) a slightly broader group that included individuals like Paul and Barnabas or the husband-wife pair Andronicus and Junia in Romans 16--these could not have been in the Twelve according to Acts 1 because they had not been with Jesus since the baptism of John, c) a more basic sense of an apostle as one who is sent for some purpose.
2. prophet (prophetes)
Most of the NT references to prophets refer to OT prophets. One of the most interesting categories though is NT prophets. When Ephesians speaks of the church being built on the foundation of the apostles and prophets, I believe it was thinking of NT prophets, not OT prophets. We can debate how much, but you could at least argue that a great deal of Christian self-understanding was generated by NT prophets. Prophecy usually involved bringing revelation about the present or near future, it would seem.
3. elder (presbyteros)
An elder is basically an "old guy." There are several special groups of "old guys" the NT mentions. The Jewish council the Sanhedrin was made up of old guys. The "tradition of the elders" was tradition on how to keep the Law passed down by old guys. And it would seem that at least many local assemblies were run by old guys, elders.
4. overseer (episkopos)
I personally suspect that "overseers" were pretty much the same group as elders. Philippians addresses the church at Philippi with its overseers and deacons. This is probably one church in the whole city, and with multiple overseers. So I'm not sure that we really have the idea of a senior pastor here--although I wonder if there were key individuals who took special leadership as well.
5. deacon (diakonos)
The word diakonos seems to have the possibility both to refer to a particular role and to ministry in general. In Philippians I just mentioned, the deacons seemed to play a fixed role in the church. It thus seems likely that Phoebe has this role at Cenchrea (Rom. 16:1). On the other hand, 1 Timothy 4:6 refers to Timothy as a "diakonos of Christ Jesus," but this is not in relation to a specific church. It thus seems likely that "servant" or "minister" here has a more general sense of one who serves the church.