I found a similar q&a here Is apt-get install method the same as Ubuntu software centre installation? but my experience suggests this is not a completely accurate answer.
I put 'ubuntu video downloader' in google, and got a hit for 'all video downloader', https://apps.ubuntu.com/cat/applications/allvideodownloader/. When I saw it had a big sticker saying it was available on the software center, I switched over to that on my desktop. To my surprise, this app was nowhere to be found. I have been googling on this issue for way too long without finding an answer, so I am posting.
I am running 15.04, and the apps.ubuntu page stops at 14.04 under 'available versions'. Is that why I am not seeing it in the Software Center? Could it have been removed from the Software Center? I see a lot of command line tools and libraries for video parsing in Software Center; are the guis just not up to speed on 15.04 yet?
So even though this specific app brought me here, it really is a broader question, as I hope the title conveys: How does Software Center really work? How is it different from, in sync with (or not) apps.ubuntu? What's included in Software Center and what isn't, why, and how would I know (other than posting here)? Thanks.