Sonic the Hedgehog, My Little Pony: Friendship is Magic, Video Games, Transformers, proper grammar, online gaming, speaking with friends, making friends, eating, using Photoshop, discussing Video Games, meeting fellow Sonic fans, Twilight Sparkle, and using Skype.
Although there aren't any formal distinctions (to my knowledge), I usually refer to the OP as a topic, and the entire body as a thread. For example, I might say the following as part of a merge message: "Your topic is a duplicate of the one in another thread."
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