This feature sends the text that a user types to Google's servers for context-sensitive spell checking, homonym suggesting, and grammar checking.
Also added in this Chrome release is a feature that synchronises the custom words added to dictionaries between different devices using Chrome.
These changes are available on Windows, Linux, and Chrome OS — Mac support is still being worked on.
For the tin-foil hat wearers who are using Chrome, the good news is that this feature is optional. But I would love to see the marketing geniuses behind Microsoft's Scroogle campaign get to work on this.
The truth of the way people are working at the present time is that Google is the easiest, generally correct spell checker available for those instances where a baked-in spell check is not available, or is too troublesome to turn on.
I would not dare to turn on a spell checker when coding (especially in vim), that would merely turn my code into a never-ending river of red underlining and blocks. But when I need to double check a word that I have questionably spelled, it is too easy to check it by simply typing the word into my browser's address bar and letting Google take care of the rest.
If Microsoft and the tin-foil hat brigade are right, then Google is already reading all of my emails anyway. The least that Google could do is make sure that they are spelled right, and that the grammar makes sense.