Jesus Christ instituted the sacrament of confession, giving his apostles the ability to "bind and loose," thus reconciling sinners with God and the Church (cf. Matthew 16:19).
To clarify, priests do not forgive sins! Only God can forgive sins!
However, Jesus uses the Church as his instrument of reconciliation in the world.
The sinner is reconciled both with God and the Church in the sacrament of confession.
Since the sacrament has that two-fold aspect to it, there exist certain sins that require extra acts of reconciliation with the Church.
These sins require extra steps, as the sin itself is grievous and is very harmful, especially to the Church. In order to repair the damage done, the penitent needs to seek reconciliation with his/her local bishop, or even the Vatican for more serious crimes.
Here are a few examples the Code of Canon Law gives for sins that excommunicate and need additional steps of reconciliation. There are many more not listed here.
Abortion used to be on this list, but Pope Francis now permits any priest to absolve individuals who procure an abortion.
1Priests who sexually abuse minors and adults
2Profaning a Consecrated Host
3Physically assaulting a priest, bishop or pope
