Machine code

Machine code is the only 'programming language' a computer actually understands. Any other programming language has to be converted into machine code, either by an interpreter or a compiler.

Why not write everything in machine code?

Machine code is not designed to be human readable. It's also extremely low-level. That makes it much more difficult to program in machine code.

Assembly

The assembly 'programming language' is essentially a more human-readable form of machine code. It's generally used to write bootloaders for operating systems or to program embedded devices.