Probability and Combinations
Key Questions

A permutations is an arrangement, so the order of its elements matters; on the other hand, a combination is a subset, so the order of its elements does not matter.
Example
All permutations of two elements from
#{a,b,c}# are:#ab, ba, ac,ca, bc# , and#cb# All combinations of two elements from
#{a,b,c}# are:#{a,b}, {a,c}# , and#{b,c}#
I hope that this was helpful.

I need more details to answer this question. This is a video my coteacher and I made. Hope it helps, if not please give me some details and I will try to do better.
http://www.frontporchmath.com/topic/probabilityusingtreediagrambagelgrabbag/ 
Combinations of items are basically subsets of the items, so the number of combinations is the number of subsets of items.
Let
#C(n,r)# denote the number of combinations of#n# item chosen#r# items at a time. Then, it can be found by#C(n,r)={P(n,r)}/{r!}={n cdot (n1) cdot (n2) cdot cdots cdot (nr+1)}/{r!}={n!}/{(nr)! r!}#
Example
Find the number of ways to choose 3 cookies out of 6 distinct cookies.
#C(6,3)={P(6,3)}/{3!}={6cdot5cdot4}/{3cdot2cdot1}=20# Hence, there are 20 ways to choose 3 cookies from 6 cookies.
I hope that this was helpful.