RFID based on the 125KHz standard is simple and robust. With a range up to 8cm, it provides a good general purpose solution. The readers are simple making them lower cost than some other solutions.
Tags come in all shapes and sizes. Independent of physical size and shape tags are available in different formats. Some tags are just a unique number that is used only for identification. Other tags have memory that may be written a read to store data in the tag.