I know that misuse cases are use cases which help identify security requirements as they highlight the user's goal in misusing the system, but what exactly is abuse cases and what is the difference or these very similar to each other?
It describes the process of executing a malicious act against a system, while use case can be used to describe any action taken by the system.
A complete abuse case defines an interaction between an actor and the system that results in harm to a resource associated with one of the actors, one of the stakeholders, or the system itself.
Contrary to what most people would naturally conclude, "Misuse Cases" require malicious intent, "Abuse Cases" do not. Misuse Case is the antithesis of the Use Case where users are not doing what they should according to established procedure. Actors are "mis-using" the system. "Abuse Case" seems to draw from the technical etymology of the term "ab-use", meaning "outside of the use".
"Misuse Case" is an intentional violation of the system by a "Mis-Actor". Misuse Cases analyse user/actor threats to the system.
"Abuse Case" does not include intent in its analysis. It is focused on the set of actions that can cause harm. Abuse Cases analyse system vulnerabilities.
But these terms are new and practitioners are defining them as we go.
Misuse may be accidental, and implies that you are using the system in a way not designed.
- e.g., run
rm -rfon your filesystem
Abuse is usually premeditated and purposeful; you can abuse a system without misusing it by doing legitimate activities in an unauthorized manner.
- e.g., DDoS attack using legitimate requests to load a page
You may be interested to read the legal text of Computer Fraud and Abuse Act (CFAA) [USA].
I found a formal definition of "computer abuse" from NIST in their Glossary of Key Information Security Terms :
Computer Abuse – Intentional or reckless misuse, alteration, disruption, or destruction of information processing resources.
However, whereas the US has an "abuse" act, the UK has a "misuse" act (Computer Misuse Act 1990) (another ref), as does Signapore (Computer Misuse and Cybersecurity Act (CHAPTER 50A)) which sounds closer to the definition of abuse.