Session replay is a great way for seeing how your users are interacting with your application. Record Replay is a great way to understand why your application does what it does.
There are several session replay tools like FullStory that are great for seeing if your users are running into issues with the checkout button. Replay is the first record replay tool for the web and is great for understanding why the checkout button turns red accidentally or doesn't add the item to the cart. Not good!
In concrete terms, session replay records the application where as Replay records the browser. This gives us access to the underlying browser mechanics so that we can build the kind of debugging tools we've always wanted.