1- donating and subscribing are different. Subscribing means you pay a monthly fee which you usually pay upfront for like a year. Donating means you paid once.
2- FO means Friends Only. You can add users to your FO group and post invisible posts t the main group. This means anything goes, you can post topics or things on FO that you can't post in gen pop without suspensions. As a donor you can have 80 friends as a subscriber you get 100.
3- if you subscribe it's a monthly and you get a few extra benefits as a subscriber but most people don't think it's worth it to subscribe. You can subscribe for 3 months and allow your subscription to run out and see if it's worth it. If it's not worth it you are automatically still a donor.

Hope that helped.