It’s a bit of software that takes your requests for web pages and serves them up from Google’s cache with compressed graphics and some other tricks to speed up page loading. Google’s servers are so fast and so widespread that Google believes the pages will load faster than if you got the page content directly from the end site.
The Google web page is typically terse, but it does specify that the web accelerator is designed for broadband connections (unlike the Sonic dialup accelerator, which attempts to do similar things for dialup connections).
Google has no obvious profit motive. There’s no charge and no advertising. If you use the web accelerator, Google learns everything about where you were surfing; privacy advocates fear the worst. It seems unlikely that Google will sell information to advertisers and spammers – it’s not their style. Perhaps Google will simply use the information to make their web searches more accurate. No one knows.