1.) A non-deliberate misspelling of the indefinite article "the". Most commonly arises from typing a message to another person over the internet too quickly.

2.) A deliberate misspelling of the indefinite article "the". Used most frequently by people who also use other deliberate misspellings or shortenings of words such as "u". Means roughly the same as "very".

3.) An abbreviation for "the extreme highest". Often, this is what is meant by people who misspell "the" purposefully as in (2) above.

Orginally started off as a typo for "the", but recently has changed a bit in meaning. "Teh" still means "the", however, it is mostly used in the same manner that spanish speaking people use the word "El".

(addendum)
Has the ability to turn nearly any word into a noun.

An expression used by idiots to helpfully identify themselves to other idiots.

Either a typo of the word "The" or a way to show extra emphasis onto the next word.

1) Typo for "the".
2) Very
3) An Asian name spellchecked with Microsoft Word.

Originally just a misspelling from people typing too fast, "teh" has become a word used by net.writers to exemplify people who are ignorant or simply not thinking clearly (perhaps because they're typing too fast).

Deliberate typo error of "The". used in online gaming discussion