Bitcoin is a digital or virtual cryptocurrency created in 2009 that uses peer-to-peer technology to facilitate instant payments.
With digital currency, there is a risk that the holder could make a copy of the digital token and send it to a merchant or another party while retaining the original.
An altcoin is any cryptocurrency or token created after the Bitcoin was developed.
Blockchain is most simply defined as a decentralized, distributed ledger technology that records the provenance of a digital asset.
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.
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.