Salesforce offers a user role hierarchy that you can use with sharing settings to determine the levels of access that users have to your Salesforce org’s data. Roles within the hierarchy affect access on key components such as records and reports.

Users at any role level can view, edit, and report on all data that’s owned by or shared with users below them in the role hierarchy, unless your Salesforce org’s sharing model for an object specifies otherwise. Specifically, in the Organization-Wide Defaults related list, you can disable the Grant Access Using Hierarchies option for a custom object. When disabled, only the record owner and users who are granted access by the organization-wide defaults receive access to the object’s records.
Roles determine user access to cases, contacts, and opportunities, regardless of who owns those records. The access level is specified on the Role Edit page. For example, you can set the contact access so that users in a role can edit all contacts associated with accounts that they own, regardless of who owns the contacts. And you can set the opportunity access so that users in a role can edit all opportunities associated with accounts that they own, regardless of who owns the opportunities.
After you share a folder with a role, it’s visible only to users in that role, not to superior roles in the hierarchy.

For more information on force.com security model, you can refer - 

https://developer.salesforce.com/page/An_Overview_of_Force.com_Security

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