Some older Macs used PDS (Processor Direct Slot) and some used Nu Bus.
NuBus would be the same on all NuBus Macs, allowing cards to be used on anything with NuBus slots (anything from Mac IIs up to 1st generation Power Macs.
PDS is different for different processors (68020, '030, '040) and for different models (the SE/30, one of the best computers Apple ever made, uses a different PDS from the 030 powered LCII and LCIII). So, make sure that you're getting a card that will in fact work with a specific model Mac.
Also, make sure you get software extensions for the card!