Cohorts are "groups" of users defined above the course context. It can be used for the following purposes:

• simplify manual enrolments - instead of adding users one by one, select cohort and enrol all users from it, one-time sync only
• cohort enrolment plugin - continuous synchronisation of cohorts with course enrolment (optionally assign role and group)
• allow synchronisation of cohorts with external systems - such as ldap

Proposal and details at: http://docs.moodle.org/en/Development:Cohorts