What is the difference between Authorize.net API and TransactionKey?

Authorize.net API Login ID and Transaction Key are unique pieces of information specifically associated with your payment gateway account. These two values are required when setting up an Internet connection between any e-commerce Web form and the payment gateway. They are used by the payment gateway to authenticate that you are authorized to submit Web site transactions. Transactions that cannot be authenticated by the payment gateway using these values are rejected.

Please check the below useful links, which explains how you can get Authorize.net API Login ID and Transaction Key:

https://support.authorize.net/authkb/index?page=content&id=A405

https://support.authorize.net/authkb/index?page=content&id=A576

If you have any query or need further assistance, feel free to revert back.

Thank you!