What is the difference between Comments and Discussions in my group?

It depends on how your group is set up, and what its intention is. Discussions are always meant to be used when multiple people are going to post thoughts about a common subject. That is what a "topic" is in the discussion area. You can have as many topics as you like, so maintaining order and keeping the topics neatly separate is very useful. Comments have a very simple structure, and you would find that any attempt to have a threaded interaction through comments is likely to be hard to follow, especially if there is more than one happening at a time.

Here are some examples of how "Comments" might be used in a group. In a private group, you could use the comments area as an announcements board. Since group members get an email notification, something like "Tonight's Meeting is Cancelled." or "Don't forget to bring samples tomorrow" is easily distributed to all team members. And anyone in the group can post a comment, so it can be pretty powerful. These types of posts are not likely to be responded to as a discussion. If you have a public group, something like "Buffalo Bills Fans," then the comments wall might just be a place for people to express themselves "They were amazing last night wooo!" or "Interview on CNN with the coach tonight!"