Pets 4 Ever

I've had to fix the chain that controls the flapper (sometimes it gets wrapped around the lever and won't let the flapper drop) a few times and have become subsequently curious about the way toilets work. The siphon action in the bowl seems pretty straightforward, as does the ballcock valve that shuts off flow into the tank once the level gets high enough. But I don't understand where the flow into the tank comes from in the first place... as far as I can tell the refill tube coming from the ballcock goes straight into the "overflow tube", which supposedly goes into the *bowl* and down the sewer to prevent flooding! How does the refill tube water get into the tank if it goes down the overflow mechanism? Conversely, how are overflows handled if the overflow mechanism leads *back into* the tank?