When we die, it is our mind/consciousness that travels and takes a new form... depending on the natural state of the mind at death will determine where our consciousness arises. When I say where I am referring to the which realm we will end up in.
So, when we pass away, most of the time we will be in a period of transition. This transition time is what we call the Bardo. Modern science has proven that the Bardo exists... take for example Dr Brian Weiss' book on Many lives, Many Masters... It tells a story of one of his patients that undergo past life regression. There are many voids between lives which she mentions as 'waiting'.
To know more about rebirths, we must understand the concept of reincarnation. I found this particular article about reincarnation which was taught by HHDL, Avalokiteshvara. Do have a read for further understanding. http://blog.tsemtulku.com/tsem-tulku-ri ... -lama.html