What does APX-hard mean?

Could you please clarify what exactly APX-hard refers to in the context of cryptography and computational complexity? I understand it's a term used to describe the difficulty of certain computational problems, but I'm curious about its specific meaning and implications. How does it differ from other complexity classes like P, NP, and NP-hard? Is it related to the concept of approximation algorithms, and if so, how? Additionally, what are some real-world examples of problems that are known to be APX-hard?