(not "The Bishop" from Monty Python)
I had the pleasure of attending the ordaining of a new elder in my local church today. So, "Who is a bishop?"
The Bible uses three words for leaders of the local church. These words are presbyter (elder), episkopos (bishop or overseer), and deacon.
The word for deacon means an attendant or "one who waits upon another". This office can first be seen in Acts 6 (although the word deacon is not used). Deacons serve the needs of the church, with little authority.
Interestingly, the words presbyter and episkopos are used interchangeably. In Titus (a key description of the office), Paul starts by reminding Titus to appoint "elders in every city" (of Crete, Titus 1:5). In verse 6, Paul starts to describe the qualities of a candidate. As he continues into verse 7, he switches to the term "overseer" (bishop, episkopos). The words seem to refer to the different roles played by a person.
An "elder" (literally older) is not necessarily older in age, but "more mature in faith". The elders are often spoken of as a group or council, making decisions together for the local church.
A "bishop" (overseer) is literally "one who oversees". A bishop is a shepherd, looking out over the flock. The bishop is looking for threats (wolves), and also seeing to the needs of the local flock.
Besides Titus, there is another good bishop passage in 1 Timothy 3 ("A bishop then must be blameless, the husband of one wife...").