In worm, you are a little worm, your body is the "o"'s on the screen and your head is the "@". You move with the hjkl keys and the arrow keys (as in the game snake). If you don't press any keys, you continue in the direction you last moved. The upper case HJKL keys move you as if you had pressed several (9 for HL and 5 for JK) of the corresponding lower case key (unless you run into a digit, then it stops).
On the screen you will see a digit, if your worm eats the digit is will grow longer, the actual amount longer depends on which digit it was that you ate. The object of the game is to see how long you can make the worm grow.
The game ends when the worm runs into either the sides of the screen, or itself. The current score (how much the worm has grown) is kept in the upper right corner of the screen.
The optional argument, if present, is the initial length of the worm.