I Hear Thunder But What Is It?

In London for the next three days the best and brightest of all thing meteorological are getting together to discuss British weather and just why it has been so bad these last few years.
Up for discussion is the jet stream, sun spots, natural cycles and global warming causing unusual weather events which is all very interesting but my question would be what exactly is thunder?
Although i am not one to argue with a 5 year old, i believe there is more to it than God moving his furniture but it seems nobody can give a proper answer.
Is thunder caused by the lightening heating the air and making it boom as it expands suddenly or by the lightening creating a vacuum as it scythes through the air and the thunder is the air molecules collapsing back together.
I have also been told that as the electricity passes through the air and it causes it to vibrate which causes the thunder sound and another theory is that the shock waves from the lightning bounce off the low hanging clouds to create a series of rumble of thunder. 
As the app at the top of my browser is currently showing a dark cloud with lightening coming from it for Wednesday and Thursday i really should find out if only so i can sound clever to a 5 year old who currently believes thunder is due to God wanting to shift his armchair closer to the window.