What exactly is.. and "assembly" language. +other stuff?

Assembler is a collection of low-level programming languages, each one fairly specific to a particular variety of computer CPU. It is about as close as you can get to programming in actual machine language code as is practical. As such, assembler is rather difficult to write but when done correctly creates extremely fast and efficient code. As such, it's usually used when processing speed is all-important and typically just for very small processes to perform a single specific task.