Main permissions setting page:
From here, you can chose what type of access you want a specific usergroup to have. In the example above, the user has "standard access plus polls" permissions though custom roles can be set up. For each permission, there are three options: Yes, No, and Never. There are other permissions that can be modified which are under the other tabs, as shown in the picture.
Yes: members of the group can access a particular feature, such as "Can start new topics." A yes permission means that members of the group can post new topics in a particular forum.
No: Members of the group are not given that feature courtesy of being in the group. However, if a user is a member of more than one group, a yes setting for a feature in one group will override the no setting for another. If a user for instance is both a "Registered user" and an "Administrator," and the "can start new topics" permission has a no setting for "Registered users" but a yes setting for "Administrators," then that user will be able to start new topics. Yes overides No.
Never: Similar to No, however, a Never setting for one group will override a Yes setting of another. Never overrides Yes.