What a Blockchain Actually Is, Simply

A blockchain is a shared, continuously growing record of transactions, copied across thousands of separate computers instead of sitting on one company's server. Each new batch of transactions, called a block, gets linked to the one before it, forming a chain stretching all the way back to the very first entry.

The append-only part matters as much as the shared part. New blocks can be added onto the end of the chain, but old blocks essentially can't be secretly rewritten, because every computer in the network holds its own copy, and a change that doesn't match everyone else's copy gets rejected.

A blockchain is a shared, ____ record of transactions replicated across many computers.

This is also why blockchains are often described as tamper-evident rather than tamper-proof: someone could theoretically try to alter a copy, but doing so undetected would require simultaneously changing the matching copies held by a huge number of independent computers, which becomes wildly impractical as the network grows.

What makes a blockchain different from a normal spreadsheet stored on one server?

A blockchain's record is typically stored on a single company's private server.