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A blockchain is a shared digital ledger duplicated across many computers. Transactions are grouped into blocks, which are cryptographically linked in a chain. Network participants verify each block using a consensus method, and once recorded, entries are extremely difficult to alter. This design lets people transact without a central authority, because the whole network agrees on and maintains a single tamper-resistant record.

The story began in 2009 when Bitcoin launched as the first working cryptocurrency, introducing blockchain as its underlying technology. It solved the long-standing double-spending problem without a central authority. Over the following years, thousands of alternative cryptocurrencies, known as altcoins, emerged, and blockchain evolved from a payment tool into a broader platform for many digital applications.

Altcoins are cryptocurrencies other than Bitcoin. The name is short for alternative coins. Some, like Litecoin, were early modifications of Bitcoin’s design, while others such as Ethereum introduced entirely new capabilities. Altcoins vary widely in purpose, technology, and reliability, and many are far more volatile or speculative than Bitcoin, so they carry significant risk for anyone tracking their prices.

Ethereum, launched in 2015, is a blockchain built for programmable applications rather than just payments. Its major innovation is the smart contract, self-executing code that runs automatically when conditions are met. While Bitcoin functions mainly as digital money and a store of value, Ethereum acts as a platform for decentralized apps, tokens, and services, expanding what blockchain can do.

Smart contracts are programs stored on a blockchain that run automatically when predefined conditions are met, without a middleman. For example, a contract could release funds only once both parties fulfill their obligations. Introduced widely by Ethereum, they power decentralized finance, tokens, and many blockchain applications. However, bugs in their code can be exploited, so they carry technical and financial risks.

Developers realized the underlying ledger could record more than payments. Blockchain evolved to support smart contracts, digital tokens, decentralized applications, and record-keeping in areas like supply chains and identity. This shift, largely driven by Ethereum, turned blockchain from a single-purpose payment network into flexible infrastructure. Adoption still varies, and many projects remain experimental, so real-world impact continues to develop.