Coax BNC cables come in two flavors, 50 ohm and 75 ohm. For experimental rigs (PMTs, DAQs, amplifiers), always buy 50 ohm cables. The 75 ohm versions are for video applications, so they’re what you’ll typically find in consumer electronics stores.
The connectors are interoperable, so it can be difficult to tell what you have. The picture above should help.
In most applications, you won’t see much of a difference if you use a 75 ohm cable where a 50 ohm cable is supposed to be. But it is an impedance mismatch, and in some cases there will be a degradation of the signal. Just buy 50 ohm cables unless you’re doing something in video with instrumentation built to take 75 ohm cables.
When ordering: “RG58″ is a common type of 50 ohm connectors, and “RG59″ is a common type of 75 ohm connectors.