Is there a "common voltage reference point" for all the circuits that connect together?
Yes, it's called ground, and it's not always a perfect reference. For example, ground loops may occur, wherein different devices see slightly different ground voltages--this can arise due to voltage drops resulting from ground currents, or directly from induced voltages in loops due to changing electromagnetic fields.
Not all devices use ground in the same way--for example, Ethernet uses small transformers at each end to send an AC signal without regard for matching ground voltages at each end. Isolated transceivers can use similar techniques to work with mismatched grounds, and optical signalling (optoisolators or fibre) have the same benefit.
Here's an example of the isolation and other signal conditioning from an Ethernet application note by Texas Instruments; note element T1 which includes transformers and chokes, and note further the high-voltage-rated series capacitor in the termination circuit, which keeps the Ethernet signals from being tied to the local ground.

Likewise, differential signalling is tolerant to some ground mismatches; while in many cases, the devices are referenced to the same ground, the differential signal ignores ground differences. Below, a signal is sent differentially, with the same common-mode noise (e.g. from noise on the ground line) seen on both lines at the receiver; the original data is faithfully reconstructed as long as the ground loop voltages are not so severe as to reach the limits of the receiver. If the ground loop is too severe, then you will need to fall back to isolated devices such as optoisolators, capacitive coupling, transformers, etc.