While roles and profiles are used to determine the user-based security, the organization wide default determines the distribution of data with the user. We use the defaults in the object to determine which people across the role hierarchy can access which objects.

Organization-wide defaults are used to restrict data in the sharing model.
In a master-detail relationship, detail records inherit organization-wide defaults from the master record. But in Lookup relationship, parent and child have different OWDs.

The four permissions in OWD are

1. Private
2. Public Read Only
3. Public Read/Write
4. Public Read/Write/Transfer(Only for Case and Lead)
5. Public Full Access(Only for Campaign)

Go to Setup –> Administration Setup –> Security Controls –> Sharing Settings –> Organization Wide Default.

Cheers!!!