What is the difference between a rejection letter and an appeal letter?

A rejection letter is the one sent by the journal editor to the corresponding author. It mentions the final status of the research manuscript. This letter primarily mentions the reason(s) for rejecting the manuscript. On the other hand, an appeal letter is a response to the editor’s rejection letter. It is sent by the corresponding author to the journal editor if the author has strong reasons to justify the publication of his/her research. An appeal letter includes some critical components to have an impact on the editor’s final decision. You can find more details about writing an effective appeal letter here.