Q. Ok. So which sins exactly can be forgiven in this world and which in the next?
A. First of all, only sins that we are truly sorry for can be forgiven at all, be they mortal or venial and -
Q. Woah! Wait! Hold up! "Mortal or venial"? Now there are mortal and venial sins?
A. Not just now - there's always been mortal and venial sins.
St. John writes in his 1st letter (5:17) "All wrongdoing is sin, but there is sin which is not mortal." Other translations say which is not "deadly" or "deathly". The three words are synonymous. Basic idea: there are some kinds of sin that kill and there are others that only injure (the soul). The Church tradition has referred to those sins that are not "mortal" as "venial" sins i.e. "that can be forgiven easily". The idea comes from the same passage of the Apostle John: "If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that." The Church has understood this to mean there are some sins that we can simply ask or pray and we'll be forgiven; as well there are other sins that require more than just this "asking" or "praying" St. John is speaking about. This "more" the Church understands to refer to the Sacrament of Confession.
In summary: venial sins can be forgiven by God by us simply praying for that forgiveness; mortal sins need more than just praying, they need Confession.
Q. What's so mortal about a mortal sin?
A. It kills the soul - or better, they kill God's life (grace) in our soul. Grace is what vivifies our souls. Without it our soul is (supernaturally) dead.
Without this divine grace in our souls, we can't enter heaven, we can't be united to God outside of us because God is not inside of us.
Happy All Saints of Africa Day!
6th November 2015