A self inking stamp is an individual single unit that contains an ink pad and a rubber stamp. It's fitted with replaceable built-in ink pads. The ink pad is pre inked so that the ink does not need to be applied by hand or stamped into an ink pad every time you want to use it. When pressure is applied to the handle a mechanism inside presses the stamp into the pre inked ink pad and ensures that the rubber stamp element is coated in ink.
Self inking stamps can be customised with your own company logo for example here or they can be purchased with images already designed eg coffee cup self inking stamps, coffee bean self inking stamps which are perfect for cafe loyalty cards. You can also buy self inking stamps for hands which are great for admission to club, pubs and bars.
The advantages of self inking stamps is that they are quick and easy to use and you don't get ink all over your fingers! The disadvantage is that they are less accurate to stamp so if you have time on your hands and you want your image to be stamped accurately into a defined space then you may wish to buy a customised stamp and separate ink pad.