Different types for different machines... But some are common to several makes of machine and are often referred to as 'univeral' type bobbins. They aren't. More properly they are Type 15 bobbins named for the old Singer Type 15 machines they were first used in. There days they come in both metal and plastic. They are one of the commonest bobbin types though far from universal.
These days with some of the top of the line machines having light sensors that tell you when the bobbin is close to empty you are best getting the type your machine came with. If you are unsure take a bobbin along with the make and model number of your machine with you when you buy new bobbins.
If your machine uses PLASTIC type 15 (fits in a bobbin case accessed from the front or side of the machine) or Singer type 66 (drop in) bobbins don't use metal ones in that bobbin case: it could cause wear.
Costumiers to the Discerning