Geth is a command line interface (CLI) tool that communicates with the Ethereum Network and acts as a link between your computer and the rest of the ethereum nodes (or network computers) just like you would download a torrent client to become part of a torrent network.
You can download Geth client from here.
With Geth client you can:
Mine Ethers (ether is the cryptocurrency fueling the Ethereum network)
Transfer funds between addresses
Create smart contracts and transact with them
Explore block history and much much more
Once you download and open Geth client, it will try to connect to main Ethereum network where you need real Ethers to perform transactions. At this stage, you should connect to Private BlockChain instead, the process is defined in the next step.