A transmitter generates a radio frequency (RF) signal, and then sends the signal through a transmission line to an antenna. The antenna broadcasts the signal in the form of electromagnetic (EM) wave into the space. When the EM wave is received by another antenna at the designated location, a transmission line passes the signal from the antenna to the receiver, and the communication path is completed.

In a word, antenna acts as a main role in wireless communication network. No antenna, no wireless communication. 
There are many types of antennas that can be classified into different categories under different circumstances.

For example:
* Classify by application: communication antenna, television   antenna, radar antenna, etc...
* Classify by frequency: VHF antenna, microwave antenna, UHF antenna, etc...
* Classify by shape: panel antenna, parabolic antenna, Omni-directional antenna, etc...