ethereum is like bitcoin but it could run dapps
on bitcoin network, we can run script, but they are limited
EVM (Ethereum virtual machine) is a network of computers (nodes and/or clients) where run smart contracts
"gas is the cost of invoking a smart contract method" :
Each invokation of a smart contract method is a transaction.
gas was initially made to deal with turing completion : to limit infine loops. It enforce devs to write efficient code, avoid DoS attack, make an attacker pay for ressource.
More the commands is complex (long lines of code) more gas you have to pay
Check current price of gas : https://ethgasstation.info/
A ethereum node is responsible to verify the blockchain.
List of nodes : https://www.ethernodes.org/network/1
A lot of different implementation of nodes, GETH and Parity are the most popular