An RFID tag will have three components, namely the chip, antenna and the substrate. The chip and antenna are held together by an interconnect. This arrangement is supported by a substrate namely silicon. what plastic you are saying is the encasement. The chip ,antenna and substrate together forms an Inlay which has got an adhesive on one side to make it stick on any object or item. These inlays are manufactured in roll and supplied to the customers.
when any human understandable information printed on one side of the inlay and other other side is adhesive, then is form of RFID is called an RFID label. for example, if u take labels for carton boxes you can see barcode information printed along with price, color and other details.This labels can just be pasted on with the help of adhesive it has got.
Tag: it comes in various form factors designed according to the customization. But basically its the chip and the antenna. This will be encapsulated with plastic or polyesters in order to make them durable and withstand higher temperatures. Tags can be in various sizes depending on its usage and applications