What makes it different from other transactions?
A normal transaction spends coins that already exist. The coinbase has a single input that points to nothing, because it creates new coins. Where a normal input would carry a signature, it has a free-form field of up to 100 bytes.
Since 2013 that field has to start with the block’s height. Miners put the extra nonce there, and pools often add their name. The very first one, in the genesis block, holds a newspaper headline: “The Times 03/Jan/2009 Chancellor on brink of second bailout for banks”.
How much can it pay?
At most the subsidy plus the block’s fees. Today the subsidy is 3.125 BTC, and fees add about 0.020 BTC to an average block (live estimate, updated hourly). Claim more and every node rejects the block. Claim less and the difference is gone for good; some blocks have done exactly that by mistake.
The outputs can go to any addresses. A pool usually pays one address of its own and then pays members separately. Since SegWit, the coinbase also carries a small output that commits to the block’s witness data.
Why can’t it be spent right away?
Coinbase coins have to wait until 100 more blocks are built on top, about 16 to 17 hours. A block can still lose a race and go stale, and its reward would vanish with it. The wait stops anyone spending coins that might disappear. The exchange called Coinbase is a separate thing that shares the name.
Sources
- Coinbase · Bitcoin Wiki
- Block chain reference: headers, Merkle trees, target · Bitcoin developer documentation
- Block chain guide · Bitcoin developer documentation
Live figures on this page come from the CloudMineCrypto API and public chain data, refreshed regularly, and are labelled where they appear. Educational only, not financial advice.