A ‘51% attack’ refers to a possible attack on a blockchain by a group of ‘miners’, who hold more than 50% of the hashrate. In such a situation the ‘miners’ have the possibility to deliberately not confirm transactions or to issue transactions twice (double-spend).
2 Factor Authentication is a double layer security measure. Most crypto exchanges use it. In order to log in, you not only need to enter a password, but also a code that you receive from the Google authenticator for example.
DAO is an abbreviation of ‘Decentralised Autonomous Organization’. This is basically an organisation that runs automatically on itself without any human interventions. The work is automatically excecuted through Smart contracts.
A block explorer is an online service to track blockchain transactions. It usually is a website where you can see all the new blocks that are being created. You can also search for transactions and wallet addresses. The best-known block explorers are blockchain.info for Bitcoin and etherscan.io for Ethereum.
An airdrop is a way to distribute coins. End users can generally get coins for free or in exchange for a small task, such as subscribing to a newsletter, sending a tweet or inviting other people via a personal affiliate link.
An altcoin is any cryptocurrency or token created after the Bitcoin was developed.
