C-j runs the command newline-and-indent which is an interactive compiled Lisp function in `simple'. (newline-and-indent)
Insert a newline, then indent according to major mode. Indentation is done using the value of `indent-line-function'. In programming language modes, this is the same as TAB. In some text modes, where TAB inserts a tab, this command indents to the column specified by the function `current-left-margin'.