What is the difference between a cohort and a group?
See #1.2 in Groups FAQ
I created a cohort and now I can't find it again!
If you create a cohort from Settings>Site administration>Users>Accounts>Cohorts and then assign it to a category, you will not see it listed on that screen. To view or edit it, you need to go to the category page and click the Cohorts link in the Settings block there:
How can teachers enrol cohorts?
By default, regular teachers are not able to enrol cohorts. However, you can give them the Cohort enroller role to allow them to do this.
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.