Venial sin is accidental sin. Like when someone cuts you off in traffic and you blurt out “You *#^%!”. It is certainly not honoring to God and neither is it a peaceful thing to do, but you also did not plan it out before hand and did not knowingly act in violation of your conscience and therefore the sin is not mortal but rather venial.
Mortal sin is when you deliberately and with full knowledge of what you are doing commit serious sin. I don’t think grabbing a cookie from the cookie jar is necessarily a mortal sin, even if it was premeditated. However, if you commit serious sin (the Decalogue is a pretty good list) and do so in full knowledge of what you are doing and you commit the sin meaning to sin then you have committed mortal sin.
The difference between the two is that mortal sin separates us from the grace of God given at baptism and if you were to die with unrepented unabsolved mortal sin then you would go to Hell.
The only normal way to be forgiven of sin is the Sacrament of Penance. When you confess and do your act of contrition, even if it is imperfect, the authority given to St. Peter and passed on to the Bishops and extended to this day is such that when the Priest absolves you your sin is forgiven.
This does not mean that God is not free to forgive sin however He chooses best; it does mean that the only normal method given to the Church by God is by confession, repentance and absolution.