A Bishop is a clergyman, in churches also in some sects and denominations a clergywoman who ranks above an ordinary priest, pastor, minister etc. It's a religious office found in the Roman Catholic, Orthodox, Anglican, Lutheran, Methodist, and other churches, an office typically seen as successors to the Apostles.
A bishop ranks above a priest or minister, but may be subject to a church leader, such as an archbishop, presiding bishop, metropolitan or pope. Major schisms have developed over whether to allow women bishops and whether to allow gay men to be bishops: Paul seems to suggest women should not have authority over men, and gays are just "icky".
Some denominations, notably the Roman Catholics, Eastern Orthodox, Anglicans, Methodists and some Lutherans believe their bishops are in a direct unbroken line from the Apostles. Therefore they believe their bishops have special Miraculous or magical power that Jesus gave the apostles. Apart from the above protestants generally believe authority was given uniquely to the apostles and reject claims of succession of power. Protestants generally believe in the priesthood of all believers  so ordination of priests and deacons is for them unnecessary. Most protestants reject the doctrine of apostolic succession.
In Catholicism, a bishop is someone holding the highest of three ranks of ordained clergy (deacon, priest and bishop). Bishops of important dioceses are called "archbishops," and some bishops (typically archbishops) are also cardinals, whose powers include electing a Pope. In the Roman Catholic Church archbishops, cardinals, even the pope are also bishops. The Pope is formally the Bishop of Rome, whose leadership of the church is based (via some now-discredited historiography) on being the successor of Peter.
In medieval and early Modern Europe, a bishopric was a feudal office that frequently came with emoluments (such as enfeoffed land) attached. The bishopric would thus sometimes be purchased for the younger son of a nobleman. The nobleman's eldest son would inherit his title, but the second son would become a bishop. This benefited the nobleman's family by giving one of its members a position of power within the church.
Bishops behaving badly
There are many Christian web sites that tell us a great deal about good things that bishops and other believers do in the name of religion. This section of the article isn't representative of what all bishops and other leading church figures do, there are many very good people who are bishops like Desmond Tutu.
This section is representative of what can happen when people think they have power from God and become corrupted by power. It's also representative of what can happen when corrupt people convince their followers they have power from God and use that as a way of controlling their followers.
Roman Catholic bishops behaving badly
Bishops have not been immune to the Roman Catholic Priest child molestation scandal. At least one former bishop admitted to molesting minors but the former Most Reverend Vangheluwe thinks what he did to his nephew from the age of 5 to 18 was only a little bit naughty. Vangheluwe says he went to Confession often, despite this his confessor/s couldn't or wouldn't convince him he was doing something serious. The church ordered him to go to a monastery though he disappeared from there, he vanished in spring 2011 and has not resurfaced since.
Very many Roman Catholic bishops and higher dignitaries up to the previous pope have been guilty of covering up priestly abuse setting a bad example copied by those lower in the hierarchy and the current pope has at times failed to act decisively over abuse. It is estimated that two thirds of "US Catholic leaders" have covered up abuse, for instance, retired New York Cardinal Edward Egan did not think the church had done anything wrong in failing to report abuse. and Cardinal Roger Mahony was stripped of his duties due to his covering up child abuse though will help elect the new pope  as will others with similar records. 
Successive bishops in Ireland failed to take abuse seriously.  Cardinal Sean Brady is under pressure to resign over failure to report and prevent child sexual abuse. Scottish bishops knew about many priests who molested children but not enough was done.  Unsurprisingly similar problems exist in other countries.
Cardinal Keith O'Brien of Scotland was yet another prize-winning homophobe accused of inappropriate acts and abusing his authority over junior priests, in two cases it appears this abuse resulted in psychological harm. One complainant alleges the degree of control a superior has over subordinate priests made it hard for him to refuse O'Brien. "He [the bishop above a priest] has immense power over you. He can move you, freeze you out, bring you into the fold … he controls every aspect of your life."  There was pressure on the complainants to keep quiet. In 2013, O'Brien resigned possibly under pressure -- accounts differ, and later admitted unspecified sexual misconduct.  Yet another alleged victim wants to sue the cardinal in court, the complainant says O'Brien groped and kissed him when he was a young student priest.  O'Brien has further been accused of groping  or trying to grope another priest as recently as 2003 in Rome at a drinks party to celebrate his becoming a Cardinal. This could be a life long habit. 
In Scotland where Keith O'Brien was active and elsewhere guidelines over sexual propriety by priests were routinely ignored. Sometimes this lead to consensual sex between priests and adult women, other times child abuse was ignored. 
It is impossible even to guess how often victims of clerical sexual abuse are successfully pressured into silence. The Roman Catholic Church sees itself as advisors over morality for the whole world but it's possible to wonder if their hierarchy really understands right from wrong.
Protestant bishops behaving badly
Independent Baptist bishop and megachurch leader Eddie Long has been accused over his lavish lifestyle, also over alleged sexual predation of vulnerable young men just over the age of consent. Provocative photos which look hard to explain innocently have surfaced  like these two, Long settled out of court.  One victim claims he was driven to a suicide attempt though confidentiality agreements may prevent the truth ever coming out.  Long's divorced former wife accused him of physical abuse including when she was pregnant. 
Is there a scandal waiting to surface among many Protestants or are the above atypical cases? We don't know yet.
- priesthood of all believers
- Just 'a Game': Outrage as Shamed Belgian Priest Downplays Child Abuse
- Child abuse bishop disappears from French monastery
- Pedophile Bishop goes into hiding
- Even Pope Francis is implicated see Pope Francis was often quiet on Argentine sex abuse cases as archbishop.
- Special Reports: Catholic Bishops and Sex Abuse
- Edward M. Egan
- US Cardinal Mahony urged to avoid conclave over abuse
- Pope conclave tainted by abuse scandal
- Catholic church failed children abused by priests in Ireland, says bishop
- Timeline: Reports into clerical sex abuse claims in Ireland
- Catholic Church in Scotland 'knew of 20 child sex abuse allegations'
- To be precise, the USA, (twice), the Netherlands France. etc.
- Stonewall unapologetic over Scottish cardinal's 'bigot of the year' award
- Britain's Top Cleric Cardinal Keith O'Brien Resigns Following 'Inappropriate Behavior' AccusationsCardinal Keith O'Brien 'accused of inappropriate acts'
- UK's top cardinal accused of 'inappropriate acts' by priests
- Cardinal Keith O'Brien resigns as Archbishop
- Top British Cardinal Faces Accusations of Committing ‘Inappropriate Acts’; Britain's Cardinal Keith O'Brien to resign;Cardinal Keith O'Brien resigns as Archbishop;Cardinal Keith O'Brien departs with an apology but no admission as Pope forces him out over allegations of inappropriate behaviour;Cardinal O'Brien complainant 'warned' of risk of damage to Church
- Cardinal Keith O'Brien sorry for sexual misconduct
- Cardinal Keith O'Brien being sued by alleged abuse victim
- Keith O'Brien faces new allegation that he assaulted a priest on the night he was made a cardinal
- Cardinal Keith O'Brien accused of sex assault while in office as Cardinal
- The sorry tale of the Catholic Church in Scotland continues to unfold
- Bishop Eddie Long launches fightback against sex claims
- Bishop Eddie Long Crowned ‘King’
- Bishop Eddie Long's Alleged Sex Scandal Victim Centino Kemp Reveals Suicide Attempt
- Bishop Eddie Long (PICTURES): Ex-Wife Alleged Abuse in Divorce Papers
- LDS Bishops Won't Be Charged With Crimes, Pinal County Attorney Says
- Brock sexual abuse case: Sins of omission rival sins of commission
- Former Archbishop of York 'did not report' sexual abuse claims