Warning !!!
Ether Currency Units
Ethereum’s currency unit is called ether
Ether is subdivided into smaller units, down to the smallest unit possible, which is named wei. One ether is 1 quintillion wei (1 * 10^18 or 1,000,000,000,000,000,000).
When you transact 1 ether, the transaction encodes 1000000000000000000 wei as the value.
Every Ethereum transaction requires payment of a fee, which is collected by the miners to validate the transaction. The fees in Ethereum are charged in a virtual currency called gas. You pay for the gas with ether, as part of the transaction.
Fees are required on the test networks too. Without fees, a test network would behave differently from the main network, making it an inadequate testing platform. Fees also protect the test networks from DoS attacks and poorly constructed contracts (e.g., infinite loops), much like they protect the main network.
Ethereum Wallet
An Ethereum wallet is your gateway to the Ethereum system. It holds your keys and can create and broadcast transactions on your behalf. for a wallet application to work, it must have access to your private keys
Control and Responsibility
If you lose your private keys, you lose access to your funds and contracts. No one can help you regain access—your funds will be locked forever.
EVM
The EVM is a global singleton, meaning that it operates as if it were a global, single-instance computer, running everywhere. Each node on the Ethereum network runs a local copy of the EVM to validate contract execution, while the Ethereum blockchain records the changing state of this world computer as it processes transactions and smart contracts.
Externally Owned Accounts (EOAs) and Contracts
Externally owned account (EOA): those that have a private key; having the private key means control over access to funds or contracts
Contract account: a contract account does not have a private key. Instead, it is owned (and controlled) by the logic of its smart contract code: the software program recorded on the Ethereum blockchain at the contract account’s creation and executed by the EVM.