Basics

Bitcoin

Imagine you have a magic notebook that everyone in the world can see, but nobody can erase. When you write "I gave Sarah 5 coins," it stays there forever. Bitcoin is digital money that works like that notebook. No bank, no middleman, just math and trust in the system.

Blockchain

Think of a chain of locked boxes. Each box holds a bunch of transactions, and every new box has a tiny window showing what's inside the box before it. If someone tries to change what's in an old box, the windows on every box after it won't match anymore, and everyone will notice. That's a blockchain.

Intermediate

Wallet

A crypto wallet doesn't actually hold your coins. It holds the secret password (private key) that proves the coins are yours. It's like having the only key to a see-through locker. Everyone can look at what's inside, but only you can open it.

Advanced

Mining

Thousands of computers race to solve a really hard math puzzle. The first one to solve it gets to add the next page to that magic notebook and earns some Bitcoin as a reward. The puzzle is hard on purpose, so nobody can cheat. That process is called mining.

51% Attack

Remember those computers racing to solve puzzles and add pages to the notebook? The rule is: whichever group writes the fastest gets to decide what the "real" notebook says. If one group controls more than half the puzzle-solving power, they can write their own version of the notebook faster than everyone else. They could spend coins, then rewrite history to pretend they never did, essentially spending the same money twice.

For Bitcoin, pulling this off would cost billions in hardware and electricity, and the moment you try it everyone can see. You'd tank the value of the very thing you're trying to steal. It's like buying a majority stake in a bank just to rob it, you'd lose more than you'd gain. Smaller networks with less computing power are way more vulnerable, which is why hashrate matters.

Sybil Attack

Your class votes on pizza vs. tacos for the party. You really want pizza, so you sneak in 15 friends from other classes wearing disguises. Your side wins because the teacher can't tell who actually belongs. That's a Sybil attack: one person pretends to be many to cheat a system that assumes everyone is who they say they are.

In crypto, creating a new identity (wallet) costs basically nothing, so one bad actor can spin up thousands of fakes to rig votes, steal airdrops, or overwhelm a network. The defense is making each identity cost something real. Bitcoin uses proof-of-work (computing power), proof-of-stake uses locked tokens. Faking 1,000 people now costs 1,000x the resources instead of being free.

The hard part: we want anyone to join, nobody to reveal who they are, and one person to get one vote. You can really only get two out of three. That tradeoff sits at the heart of a lot of blockchain design decisions.