Q: Basic question about synching photos from iOS to OS X

I don't seem to understand the concept of synching an iOS device to a OS X device. When I connect my iPod to my iMac, I use iTunes to sync them. My understanding of "sync" is that both devices place data on each other that the other lacks. That data is then accessible on both devices. But after synching, I cannot find the iPod data, photos in particular, anywhere on the iMac. I understand that one can IMPORT selected photos or other data into a designated app but that is a separate task from synching. Does synching from iOS to OS X mean that it's basically a backup for the iOS device, but that that data is not accessible on the OS X device?