「ので」: "X happened, therefore Y happened".
Implies that there is correlation, but not necessarily causation}}
ちょっと忙しいので、そろそろ失礼します。 I'm a little busy, therefore I'll be making my leave soon.
「から」: Y happened specifically because X happened.
時間がなかったからパティーに行きませんでした Because there was no time, I didn't go to the party.