About Bitcoin and "the Blockchain"
Since its inception in 2009, blockchains have not only provided a unique solution for recording monetary transactions but also have become a huge platform for hosting secure and decentralized applications.
The blockchain idea was initially conceptualized by Stuart Haber and W. Scott Stornetta during the nineties. Satoshi Nakamoto, the founder of Bitcoin project, implemented the first blockchain network and introduced a cryptographically secure currency called Bitcoin in 2009. Into the blockchain, you could publicly store any kind of entry, including but not limited to a simple payment history.
Once you write something to the blockchain, it cannot be erased. Hypothetically speaking, if you had to do that anyhow, you would need to convince at least a half of the parties involved in the blockchain to agree with you which is extremely unlikely.
In other words, there is no single or a centralized party who runs, participates in, or hosts the Bitcoin's blockchain. Similar to legacy banking and payment solutions, blockchain keeps a full record of who owns what. Unlike traditional centralized networks, you can freely join blockchains and help securely record, verify, and process transactions. The more there are participants in the blockchain, the network becomes more available, secure, and stable.
Ethereum Smart Contract Platform
Ethereum project was conceptualized by Vitalik Buterin who was also a co-founder of Bitcoin Magazine. Vitalik Buterin authored several articles related to Bitcoin and blockchain network and has contributed to various open source projects including Bitcoin. Vitalik first introduced Ethereum project in his white/yellow paper in 2013. He suggested that blockchain would need a scripting language for hosting applications.
Ethereum, also being a decentralized platform like Bitcoin, runs smart contracts which are basically applications that run exactly as programmed without any possibility of downtime, censorship, fraud or third-party interference. Over the past years, Ethereum has matured and the smart contracts hosted on the Ethereum blockchain and are now highly secure by design.
Ethereum was the first of the “next gen” blockchains and as such has had a number of newer currencies based on the same platform, which have in turn helped with Ethereum's success and mass adoption. Several Ethereum-based independent networks and application platforms have been tested and evaluated by companies like Microsoft, VISA, and IBM.
Smart contracts are final and irreversible. You can trust smart contracts because you can look and verify the code. You don't need an agent or any third-party to sign the contract anymore. Once written, it's extremely difficult if not impossible to replace a smart contract. Since smart contracts fully enforce the initial agreement, one can not simply back out from the promise once given.
Moreover, another major benefit of smart contracts is that they execute automatically, reducing human involvement and saving time. Except for bugs in the code, human errors cannot simply occur.