Mathematics
Normal Subgroups and Quotient Groups
Quick fact
A subgroup N of G is normal if and only if it is the kernel of some group homomorphism from G to another group. This ties the 'ideals' of group theory directly to the structure of homomorphisms.
Why this is interesting
You know how you can divide numbers to get smaller, simpler ones? In group theory, there's a way to 'divide' a group by a subgroup—but only if the subgroup is special enough. What makes it special?