RFID tags can be active, passive or semi-passive.
Passive tags do not emit radio waves unless they are exposed to the electromagnetic field of the certain frequency and power. Then they start to transmit their signal in the air.
Passive tags are smaller and lighter than active tags but have a shorter communication range and require a high-powered reader. Passive tags are generally read-only and as such, once it is programmed with data, it cannot be modified. They may live for years. Their cost mostly is around 20 cents, but it may reach $2 for metal tags.
Semi-passive tags use a battery to run the chip's circuitry, but communicate by drawing power from the reader.
Active tags are more sophisticated and have an on-board battery which provides them with power to transmit over a greater distance. They also can store much more data. They are bigger than a coin, thicker, and their battery lives a few months. They cost around $5-$25 depending on the frequency band used.