Identity Policy – Assigned to a user group or role, can be either 1.) in-line with the identity, 2.) User Managed Policy (defined by an User with the permission to do so), 3.) AWS defined managed policies.
Resource Policy – Assigned to an AWS resource like S3 or EC2. Resource-based policies are inline policies. There are no managed resource-based policies.
When applying policies to a role, a role will contain both a Trust Policy and and Identity Policy:
Trust Policy – Is a specific type of resource policy for IAM Roles – a trust policy defines who can assume a role, and under what circumstances / conditions:
Session Policies – are advanced policies you pass as a parameter when programmatically creating a session for a role or federated user – see under Session Policies:
https://docs.aws.amazon.com/IAM/latest/UserGuide/access_policies.html