No two people speak exactly the same language.
An idiolect is an
individual dialect - the
idiosyncratic way that a particular person speaks.
It is the subset of language that they use and how they pronounce it. This is not just words known, but preferred word choice, grammar used and commonly used turns of phrase.
In online texts, a related area or perhaps an extended meaning would be the characteristic layout and formatting used, and other statistics such as average word, sentence and paragraph length. In the absence of proofreaders it would also include characteristic errors.
Your idiolect identifies you. Given a large enough sample of writing, there are fairly sophisticated programs that can identify the author out of many candidates with a high degree of certainty, by comparing it to texts with known authorship. 1
Professor Don Foster, a Shakespearean professor at Vassar College who identified an anonymous poem as by William Shakespeare 2 by this technique, later identified Joe Klein as the author of the book Primary Colors using the same method.
The Fist of a telegraph operator is a similar identifying style - not of word choice, but analogous to pronunciation.
1) I'm sure you could defeat this if you tried very hard: Create an alternate persona and pick a writing style for them. Compare your new persona's writing to your normal work using this kind of software and deliberately change the new writing in consistent ways until the software doesn't report an match anymore. Perhaps you could keep it up for a few pages. For more than that, it would be less hassle to get a ghostwriter.
2) By "by Shakespeare", I mean by the same person who wrote the rest of Shakespeare's works and who we may therefore refer to as "Shakespeare" no matter who they really were.
Sources:
Identifying the author of primary colors:
http://www.infidels.org/library/magazines/tsr/1998/4/984front.html