Interpreter:

An interpreter is a program that translates each statement in the programming language into machine code and runs it.  Such an arrangement means that to run the program one must always have the interpreter available.  

e.g Basic , Prolog, Perl are languages that are typically interpreted.  

Programs in any language can be interpreted or compiled.  So there are basic compilers available as well.  Compiled code runs faster and does not need the compiler at run time, whereas interpreted code is slower and needs the interpreter every time the program has to be run.