What is the main difference between a Compiler and a Transpiler?

Compiler - compiles code to a lower level code.

Example:

• "Developer code" -> "Machine code"
• PHP -> C
• Java -> bytecode

Transpiler - compiles code to same level of code/abstraction.

Example:

• "Developer code" -> "Another developer code or version"
• JavaScript ES2015+ -> JavaScript ES5

They're essentially the same: take source code and transform it to something else.

The difference is that compiler usually produces a directly usable artifact (executable binary of some sort). Example: C (produces binary), C# (produces bytecode).

Whereas transpiler produces another form of source code (in another language, for example), which is not directly runnable and needs to be compiled/interpreted. Example: CoffeeScript transpiler, which produces javascript. Opal (converts ruby to javascript)