Summary

Cadence transactions use phases to make the transaction's code / intent more readable and to provide a way for developer to separate potentially 'unsafe' account modifying code from regular transaction logic, as well as provide a way to check for error prior / after transaction execution, and abort the transaction if any are found.

The following is a brief summary of how to use the prepare, pre, execute, and post phases in a Cadence transaction.

transaction {
    prepare(signer1: AuthAccount) {
        // Access signing accounts for this transaction.
        //
        // Avoid logic that does not need access to signing accounts.
        //
        // Signing accounts can't be accesed anywhere else in the transaction.
    }

    pre {
        // Define conditions that must be true
        // for this transaction to execute.
    }

    execute {
        // The main transaction logic goes here, but you can access
        // any public information or resources published by any account.
    }

    post {
        // Define the expected state of things
        // as they should be after the transaction executed.
        //
        // Also used to provide information about what changes
        // this transaction will make to accounts in this transaction.
    }
}