its basicly when you divide the whole number up until it can't be divided any further.
The prime factorization.
If completed correctly, the bottom branch of a factor tree will be the prime factorization of the number at the top.
factor tree
With a factor tree, yes.
13 is already prime, no tree required.
A factor tree
2x2x222
8
50 25,2 5,5,2
When you want to find the prime factorization of a composite number.
15 23
22 2,11