The world for overseers is 'episkopos', which means 'those looking on'. It is translated 'bishops', and 'overseers', referring to the office itself. The word for elders is 'presbuterous', and means seniors or older ones, and refers specifically to the persons who occupy the office. Note that the word 'elder' can mean 'older ones' in a general sense, depending on the context, as in 1 Peter 5:5; "Likewise ye younger, be subject to the elder". But in other places 'elders' are those who occupy the office of an overseer, as in Acts 20, where Paul spoke to "the elders of the church", saying, "...all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God" (Acts 20:17, 28). Those who fill this office have the care of the local assembly, to feed and shepherd them. We find that the office of an overseer was local; "elders in each assembly" (Acts 14:23), contrary to the Church's practice of setting up "bishops" who preside over vast regions. More details on elders will follow. Elders are always mentioned as a plurality, never a single man over an entire congregation.
It is helpful to note that there is no mention of elders or bishops in the Thessalonian assembly. They were a young assembly, and Paul had not visited a second time. In Acts 14:21-23 we find it was Paul’s habit to appoint elders in a local assembly on a subsequent visit. We only ever read of apostles or special delegates that were given authority to appoint elders (Titus 1:5); never the local assembly. But this passage shows that even without the official relationship, there were those the Holy Spirit was stirring up to do that work; "those who labour among you, and take the lead among you in the Lord, and admonish you" (1 Thess. 5:12). Note that 1 Tim. 5:17 couples "taking the lead" with elders. The exhortation is to recognize those who labored among the saints and took the lead among them. There were overseers mentioned in Ephesus and Philippi, but not in Rome, Thessalonica, or Corinth. It is important to have oversight, and for the assembly to recognize it, in order to have peace in the assembly. Even without official appointment, we can recognize and esteem those who do the work of an overseer. Notice that there is no mention of elders in Corinth, because it was a young assembly, and Paul had only visited once. Yet even there, in the first epistle, there was "the house of Stephanas" who had "devoted themselves to the saints for service", and that the saints should "be subject to such, and to every one joined in the work and labouring" (1 Cor. 16:15-16). What a tremendous help this is to us, who live centuries after the apostles and their delegates passed off the scene. God is still raising up leaders in local assemblies, even though they have no official title.