FASTag is an electronic system for toll collection system in India, operated by the National Highway Authority of India (NHAI). It is the fastest way to overcome queuing at toll gate to pay fee. It employs Radio Frequency Identification (RFID) technology for making toll payments directly from the prepaid or savings account linked to it or directly toll owner.

Ref: https://en.m.wikipedia.org/wiki/FASTag

Fastag is consist of three main components, namely

1. RFID tag (at customer side) 
2. RFID Reader (mounted at toll gate)
3. Internet server for payment

1. RFID tag: This is a small wireless device that design to transmit small code data by its Radio Frequency transmitter. It may be battery powered or  electromagnetic coil based self power generation. But, here it will battery powered, because of long distance transmission. 

2. RFID Reader: RFID Reader is a fixed device that is mounted somewhere near the passes of vehicle. It's work is to read RFID code from RFID tag. This reader is connected to internet.

3. Internet server: Internet server is used to connect Fastag system to customer online payment account.