an anonymous personality calling himself Satoshi Nakamoto first launched bitcoin in 2008. Since then over two thousand other cryptocurrency experiments have been launched. EXPAND for list of currencies.
Bitcoin is the first cryptocurrency. It uses digital signatures to identify and secure transactions denominated in its native currency. It also prevents account owners from double spending balances using a technique called proof of work to achieve consensus about which transactions are valid. This results in a ledger of transactions recorded in a peer to peer distributed database called a blockchain. Transactions are implemented via a simple but secure scripting language allowing for the first programmable money.
Ethereum extends bitcoin by incorporating a more complete, easier-to-use, but less secure programming language to implement smart contracts. Ethereum trades security for ease of use because of this developers have chosen it as the platform on which to launch the first decentralized applications sometimes with associated initial coin offerings.
Ripple is actually a company, but they created a cryptocurrency called XRP. XRP is pre-mined meaning all coins were created at once and then distributed or sold to various parties. XRP solves the double-spend problem differently than bitcoin, there is no proof of work. Whereas Bitcoin's origins are philosophically opposed to existing financial infrastructure and bitcoinattempts to route around it, Ripple and XRP are aimed at improving existing financial infrastructure via cryptocurrency tech.
Bitcoin Cash was created when Bitcoin developers had a dispute about the best way to scale the network to meet rising demand. Some developers and miners hard-forked the bitcoin blockchain by increasing the number of transactions that a single block can support in the process creating a new ledger with a very similar set of rules.
Cardano is probably most similar to Ethereum in that they are attempting to be a platform for decentralized applications but they use a consensus mechanism called proof-of-stake instead of proof of work. They also try to use very strict development standards like formal proofs and peer review to make sure they can meet the scalability challenges faced by successful cryptocurrency networks.