The Enter key does send a CR character (carriage return, Ctrl+M, numerical value 13 = 0x0d = 015). You can see that at a shell prompt or in Vi insert mode by pressing Ctrl+V then Enter: Ctrl+V is a keyboard shortcut to enter the next character literally in the kernel's built-in terminal driver as well as in many terminal-based programs including typical shell and Vi(m).
In Vi(m) insert mode, the character Ctrl+M is bound to the command “insert line break”. Vi reacts to most input characters by inserting that characters, but there are a few exceptions, most obviously the character Ctrl+[ which is what the Esc key sends.
In a text file, a line break is represented by a LF character (line feed, Ctrl+J, numerical value 10 = 0x0a = 013).
Pressing Ctrl+J in Vi would actually have the same effect, but you could bind the two keystrokes to separate commands if you wanted. You can observe a difference between Ctrl+J and Ctrl+M in command mode: Ctrl+J simply moves the cursor down to the next line whereas Ctrl+M moves the cursor down to the next line and also moves it to the first non-blank character.