What is the difference between account team role & contact roles?

Answer: An account record can have an Account Team working on that account. Account team role decides what role any Account Team Member plays in that Account Team.

Explanation:
‘Account Team’ is a related list on any account record. ‘Contact Roles’ is
also a related list on an account record which is distinct from
the ‘Contacts’ related list. Any contact can be added to the
‘Contact Roles’ related list along with specifying what role that
contact plays on that account.