This is a confusion in terms. Personally I blame Sun. An interpreter IS a form of compiler, it is the term used to refer compilation at run time
No, sorry. A compiler is, in theoretical terms, a partial application of an interpreter to a program. In practical terms, a compiler transforms the input into some other form, which is then executed, whereas an interpreter executes the input directly. JIT compilation is still compilation. A just-in-time compiler is the term given to compilers that produce their output just before it is executed, as opposed to ahead-of-time (AoT) compilers, which produce it all up front, even if some paths are never executed.
There's some complication, because most environments that do JIT compilation also include interpreters that gather profiling information to incorporate into the JIT compiled code and to improve startup times. JavaScript implementations, in particular, often spend a reasonable amount of time in the interpreter because most web pages contain a load of JavaScript that's only run one or two times and the time taken to compile it is more than the time saved to execute it. Some have multiple compilers - JavaScriptCore from the WebKit project has an interpreter and three different JIT compilers that have different points in the space between compilation time and execution time - they'll recompile hot paths multiple times as they're executed more, with more optimisation each time. The key difference between the interpreter and compilers here is that the compilers are each invoked once on a segment of code and it's then executed without involving the compiler. The interpreter is involved every time the bytecode is run. It reads a bytecode and then jumps to the segment of interpreter code that executes it and then returns. The compiler takes a sequence of bytecodes, generates a fragment of native code to execute them, and then this fragment is combined with other fragments to produce a running program.
The shader compilers in drivers, however, are not JIT compilers. They are AoT compilers that are invoked at load time - often at install time. They don't compile the code just before it's run, they typically compile it once and cache the result for multiple invocations of the program. Some drivers (Windows and Android come to mind) have a mechanism that allows you to do the compilation at install time. Unlike most JIT environments, graphics drivers don't tend to use run-time profiling for optimisation, the bytecode exists solely for the purpose of providing an ISA-neutral distribution format.