A transaction is a signed instruction sent to a blockchain by whoever has the private key associated with an address to move cryptocurrency from that address to another address, or to execute a smart contract, also known as a decentralized software programme, or dapp.
Transactions are created by apps like Emerald Wallet, where users have their cryptocurrency balances, private keys, and blockchain addresses.
When users wish to send a transaction to move money in a blockchain or to use a smart contract or dapp, they write the appropriate address of the receiver or the smart contract or dapp, they select what fee they will pay for that transaction, and they sign the transaction with their private key, which corresponds to the address of the user.
Once the transaction is written and signed inside Emerald, then the user sends the transaction to the blockchain in question through the app.
Then, Emerald processes and sends the transaction through its blockchain nodes, which are machines that form part of the wider blockchain networks.