SIM or Subscriber identity module is an integrated circuit embedded in a small card used widely in mobile devices. SIM card stores international mobile subscriber identity number (IMSI). This number is used to identify the subscriber on a particular network.
Micro SIM- is a SIM card which is small in size than a normal SIM
Nano SIM- is the smallest of all kind of SIM card available.
Virtual SIM- here there is no physical SIM but the functionality will be similar to normal physical SIM. This is to be created online. Only few network providers are currently offering virtual SIM card.