RFID based on the 125KHz standard is simple and robust. With a range up to 10cm, it provides a good general purpose solution. The readers are simple giving them total cost that is lower than many 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 to and store data in the tag.