Usually when I talk about or hear someone use the term 'Master Link' they mean something like this:
They are links that can be separated without the use of a specialized chain tool to push a pin through the rollers. There are others besides those shown, one of the more common being an SRAM Power Link which looks like this:
If you don't have a link like those that is visibly different, what you most likely have is a Shimano style HG or IG chain that requires the use of a chain tool to remove a pin from the roller. Frequently the pins will have a little indentation where the chain tool seats. When you break the chain with a chain tool, you normally reassemble it with a replacement pin that looks like this:
After the chain is back together, you break off the insertion portion of the pin with a pair of pliers.