What is the difference between a cohort and a group?
A group exists only within a course. You might have a class 10B for example which you want to enroll in course 1 and course 2. The group would need to be created in both courses and its members enrolled separately in both courses. Cohorts are groups of users created by admin which can then be brought in "en masse" to courses. So if class 10B is created as a cohort by admin, then it can be selected in those courses where enrolment via cohort has been enabled. (For more information on creating and using cohorts, see here: Cohorts) Once members of a cohort have been added to a course, they can be included in a group, either by using the "auto create groups" feature or by adding the members to an empty or pre-created group.
Read more in Groups FAQ
Can I synch cohorts with LDAP groups?
This is not currently available as standard. However, the tracker issue for this (MDL-25011) offers a patch which works on versions up to and including 2.3. It has not yet been tested on 2.4.
Where can I practise with some ready made cohorts?
I want to enrol a cohort of students into a category
This is not currently possible. Please see MDL-36951 for the reasoning behind this.