Typically terminal control escape sequences. Example (here for bash, a similar version works in tcsh): Type Control V, and then a cursor right key. What you will see on the screen is "^[[C". Here's why: When you press the cursor right key, the terminal (emulator) actually emits the control sequence Esc [ C. That's because terminals emit the ASCII code of the key that is pressed (for example "a" = 0x61 when you hit the lowercase a key). But there are no ASCII codes for most of the function keys on the keyboard, only for the "displaying" characters. So those function keys like cursor control, the F-keys and the block of 6 keys above the cursor control keys emit escape sequences, a set of characters that starts with the Escape character. And the shell knows to not echo those to the screen, but take an action based on them.
When you press Control V, you are telling the shell: Take the next character that you see literally, as if I had typed it, and put it into the command line. That removes the Escape from "cursor right", leaving the "[C". And since Escape is echoed by the shell as "^[" (since it is really is Control-opening-square-bracket), you see "^[ [ C".
The same works the other way. When emacs or vi do complicated drawing on the screen, they use lots of complicated escape sequences. For example: Esc [ 31 m means: Print the next text in red. Esc [ H means: start drawing at the beginning (top left corner) of the screen. And so on. If you confuse the terminal emulator (for example by printing another escape character, or putting it into a strange mode), sometimes these escape sequences that are being sent to the terminal become visible.
If you want to figure out what these escape sequences are, search the web for "ANSI vt100 escape sequences" or something like that.