Leet, also known as “1337” is a writing system developed for the English language which gains some of its value in being difficult to understand — but not too difficult. It is in some respects a code-substitution cipher, where glyphs — especially numbers — are used in place of letters which look similar. For example, “3” is frequently used instead of “E”, “7” in place of “T”, etc. (This makes Thaana‘s use of numerals from other writing systems look somewhat less odd.)
Combinations of glyphs are also substituted for single Latin characters sometimes, e.g. “|<” for “K” or “/\/\” for “M”. Unusually among writing systems, there is a great abundance of “legal” ways of writing a character. Just a few of the ways to write”M” include “
|\/|". "44", "IYI", "AA".
Leet is also somewhat logographic, using rebus principles. For example, “banned” can be written “b&”; “great” can be written “gr8”.
There are number of other “misspellings” and non-standard grammatical forms used to transcribing standard English to Leet in the service of making the communications less understandable.
Note: I put “USA” down for the country of origin in the title, but that is undoubtedly an over-generalization. Leet developed in the BBS culture, a majority of which probably originated in the US, but Leet is an international ad-hoc collaborative evolutionary endeavour.