RFID tags classification
The RFID tags have different classifications according to different standards.
Active tags and passive tags
        Practically, the tags work when it has been provided electrical power, although it requires a little power. They can be divided into active and passive tags according to different ways of obtaining electrical power.
        The active tag has their internal battery to supply enough power. It has high reliability, and long distance of signal transmission. In addition, it can control the period of use or times of use and by designing battery life. Thus it can be applied to where it has to restrict data transfering or data access. Main disadvantages of the active tag is that it is expensive, large in size, and life of the tag is restricted. And as the tag consumes power, the transmission distance is getting smaller and smaller, which will affect normal operation of the system.
        The passive tag does not have an internal battery, which means it has to rely on outside power supply. The typical devices in the passive tag to get power is the antenna and coil. When the tag enters system working area, antenna will receive the electromagnetic wave, and the induced current will be resulted in the coil, which will charge the capacitor. The capacitor voltage will become the working voltage after stabilization. The use of passive tag is unlimited, and it is always applied to where it requires frequent access of information. In addition, passive tag supports long-time data transmission and permanent data storage. The main disadvantage of passive tag is that its data transmission distance is shorter than active tag. That’s because passive tag relies on the external electromagnetic induction power supply, which is relatively weaker, and the data transmission distance and signal intensity is restricted. Thus it requires more sensitive signal receiver to conduct reliable recognization and reading. However, its price, demension and easy-to-use determines that it is the main stream of RFID tags.

The RFID tags that will be mentioned in the following paragraph are passive tags.
Read-only tag and read-write tag
        According to different types of internal memories, tags can be divided into read-only tag and read-write tag. Read-only tag only has ROM (Read Only Memory) inside. ROM stores tag information. This information was written in by the manufacturer in the production process. Also special coding information can be written in the tag by the user before use according to specific application purpose. The information can be written in once only, while it can be read for many times. There is a buffer memory in the read-only tag, which temporarily store modulated information that is waiting to be send. Read-only tag is generally small in capacity, and can be used as identification tag. In the identification tag, one number or strings consist of numbers and letters are stored, which is the KEY to access the database in information management system. What stored in the identification tag is only the identification code, it identifies specific items, such as people, objects and places. Specific information on identified items can only be accessed in the database connected to the system.
        In the read-write tag, besides ROM and buffer memory, there is a non-active programmable memory. Such memory is generally EEPROM (Electrically Erasable Programmable Read Only Memory). In addition to data storage, it allows the erasure of original data and data re-write. The read-write tag might have RAM (Random Access Memory) to store temporary data in the tag reaction and data transmission process.
        Read-write tag is generally greater in capacity, this type of tag is programmable by user. In addition to the identification code, there is also other information about the identified item, such as production information, counterfeit-proof verification code etc. In practical applications, all of the information about the identified item are stored in the tag, and can be accessed when reading the tag. Access of information does not require connection to database any more. Moreover, when reading the tag, it can control the reading out of data according to specific application purposes, to achieve different data access in different situations.
        Generally, the tag ROM stores manufacturer code and no repeated sequence code. Each manufacturer code is fixed and different from each other, while the sequence code of each product of each manufacturer is different as well. Therefore, every tag has a unique code, which is stored in ROM, that is to say, the tag cannot be copied, and is the base point of counterfeit-proof.