Stephen O’Grady has written one of his patented Q & A pieces about Google Gears. Here’s his summary of the Google Gears’ guts:
Gisting this down, you essentially have a local web server (which one, does anyone know?) that serves cached content out of a SQLite database, with a threading mechanism that ensures that abnormally strenuous tasks – say, downloading and caching several thousand email messages – doesn’t bring your browser down while it waits. It’s pretty straightforward, actually.