Difference between Compiler, Interpreter And Assembler?

Compiler -- reads human-readable source code, produces machine-executable binary code. Examples are C, COBOL, Java, etc. Easiest for humans to program, but does not always produce the most efficient executables.

Interpreter -- Reads human-readable code, line at a time, and produces and executes machine instructions "on the fly". Example is good old BASIC. Good for testing, but is VERY slow.
Assembler -- Converts machine-manipulation coding directly into binary machine instructions. Produces the most efficient executables, but is the most difficult (for humans) to work with.
Best of all worlds (my opinion) is C on Unix, for speed, ease of use, program efficiency.