Record-Level Security

In Force.com, individual data records within an object are secured through a combination
of three concepts:
1. Record Ownership: All records except those on the child side of a Master-Detail
relationship have a single named owner. Record owners are individual users or
groups of users. Ownership of a record can be transferred manually to another user
or group.
2. User Groups: Users can be organized into flat lists and placed in a hierarchy.
Groups can contain individual users as well as other groups.
3. Sharing Model: The sharing model consists of two parts, organization-wide
defaults and sharing reasons.The organization-wide defaults can be configured to
lock down all records by object, regardless of their owner. Sharing reasons selectively
override the defaults to allow access based on record ownership or arbitrary
criteria.