This is one of the fundamental yet confusing doubts one has in this very topic..
I will try to let u understand through a simple example..
Problem : How many ways can we select a cricket team of 11 players out of a squad of 13 players.
Now here say we have selected a team of 11 players..Hence we are concerned about selection of players only but not arrangement among them..Hence this becomes the problem of combination.
Here no of ways = 13C11 only.
Now lets modify the above problem to make it a permutation problem..
Problem : How many batting orders can be made from a cricket team of 11 players out of a squad of 13 players.
Now here arrangement among the selcted players matter..[ Say Virat Kohli batting at no 4 instead of no 3 , so it will be considered as different only..:) ]
Here no of ways hence = Selection of 11 players from 13 player squad * Batting orders possible among the selected 11players
= 13C11 * 11!
= 13P11
Hope this helps..:)