Symbols are a large part of Leet, including the use of Arabic numerals, ASCII characters, and more. This is applicable to any kind of Leet (except for 'soft leet,' or 'chatsp33k,' which use mostly letters).
Here are some examples of three versions of Leet (Middle Leet, Poetic Leet, and Hardcore Leet). Note the amount of ASCII characters used in each example.
Middle Leet Edit
"PH33R M3! 1 R TEH H4XX0RZ! J00 C& ST0P M3!"
- Note the slight readability.
Poetic Leet Edit
I R teh haxxorz,
c& j0 C?
I R teh maxterz,
- Note the use of rhyme and rhythm. See Poetic Leet for more information.
Hardcore Leet Edit
3y3 ® t3|-| ü83® |-|/-\><cks0® uv |\|008 p\/\/n463 & 1337|\|355!!!!11oneone shift+1
- Note the extensive use of the /, \, -, and number keys.
ASCII, Arabic, and Latin symbols are used very extensively in Leet.