Sin is an offence against God. It is a failure to love God or our neighbour. It is any thought, word, deed or omission which offends God or our neighbour. It is an offence against reason, truth and right conscience.
Sin always involves some act of disobedience or selfishness.
The original sin was committed by our first parents, the first human beings created by God. When they disobeyed God they turned their backs on him and lost the spiritual gifts and blessings which God had given to them - no longer able to share the life and love of God. The original sin has left the human weakened with a tendency towards sin. Actual sins are the sins we ourselves commit. Actual sins can be serious and grave and these are called mortal sins because we reject God's life and love and we could lose God forever.