I want to know if Objective-C is an interpreted or a compiled language.
It is neither. Objective-C is a programming language. A programming language is an abstract concept. A programming language is a set of mathematical rules and definitions. Programming languages aren't compiled or interpreted, they just are.
Compilation and interpretation aren't properties of a programming language, they are properties of, well, a compiler or an interpreter (duh). Every language can be implemented by a compiler and an interpreter, and most languages have both compiled and interpreted implementations. In fact, the majority of modern language implementations utilize both an interpreter and a compiler in the same execution engine for maximum performance.
For Objective-C specifically, I know of three implementations: gobjc, clang and oscompiler, but a quick Google search turned up two more. Of those five implementations, three are compilers and two are interpreters.