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.
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).
Launched in 2015, Ethereum is the world's programmable blockchain. Like other blockchains, Ethereum has a native cryptocurrency called Ether (ETH). ETH is digital money. People all over the world use ETH to make payments, as a store of value, or as collateral. But unlike other blockchains, Ethereum can do much more.
Cold storage refers to storing cryptocurrency on a place where the private key cannot be accessed via the internet. This can be done on a hardware wallet, paper wallet or software wallet in an offline environment.
An altcoin is any cryptocurrency or token created after the Bitcoin was developed.
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.
