What Is a Block?

You send a transfer and the app shows “pending” for a few minutes before it finally says “confirmed.” What's happening in that gap is that your transaction is waiting to be included in a block — a batch of transactions that gets verified and locked onto the network's record all at once.

How blocks link together

Transactions don't get added to a blockchain one at a time as they happen. They sit in a temporary queue until the network groups a set of them into a block, checks that they're all valid, and appends that block to the end of the chain. Each new block contains a reference to the block before it, which is what makes the sequence a chain — you can trace any block back through every one that came before it, and altering an old transaction would break that chain of references in a way the rest of the network would immediately notice.

What confirmation actually means

When people say a transaction is “confirmed,” they mean it has been included in a block that the network has accepted. It's often described in terms of how many blocks have been added after it: one confirmation means it's in the latest block, several confirmations mean multiple blocks have since been built on top of it, making it progressively harder to reverse. That growing depth is what gives people confidence a transaction is settled, rather than any single moment where it flips from “pending” to permanent.

Why block timing affects how long a transfer takes

Different networks produce new blocks on different schedules — some roughly every few seconds, others every several minutes — and a block also has a limited amount of space for transactions. If your transfer arrives when the queue is long, it may wait for more than one block cycle before it's included, and wallets often let you pay more to be prioritized sooner. That combination of block frequency and available space is the main reason a transfer can take anywhere from seconds to much longer, depending on the network and how busy it is.

For what a blockchain is doing at the level above individual blocks, see What Is a Blockchain?, and for how to sanity-check a transaction before it's ever sent, see Transaction Verification.