A Blog is more personal, you can create and sort your entries into categories. You can also moderate or deny any comments to your entries. This could be used to share something with us, or write about your dogs or how your training is going.
A thread is more about 'discussing' or 'debating' issues. In a thread you do not really have any control over who replies and thus the subjects often get diverted from the original thread.
Personally, if I wanted to share something about my dogs, I would do it in the Blogs section. But if I wanted to discuss Shaping techniques, I'd probably write it in the Lounge. :dogsmile:
Hope this helps,