Note: Roll-up summary fields per object has been increased to 25 (previously 10) as of the Winter 16 release.
For instance, opportunity to account is a lookup field.
However, you can establish roll-up summary fields on account to summarize opportunity data. And when an account is deleted, all related opportunities are deleted as well.
You don’t need to know the nuts and bolts of every standard relationship, but it is important to know that not all standard relationships are equal.