blogs are a personal dairy like things where you write about the things you want to write about aqnd have people comment on it.

forums are open to everyone to make threads about what they want to chat about. a forum is like a permanent chat where the chat remains forever.

groups are like forums but private or public that are of an interest to one group of people, or private invited group
i.e.
admin group
moderator group
trainspotting group