You have a right to know what data is being collected.
Google is famously a data-driven organization. They collect tons of data anonymously from users and use it to improve their products and services. For example, they may have different link placement for different users, and track how long it takes for user one to visit the link versus user two. That data might then be used to determine which placement is more ideal and user-friendly.
During the initial setup of your Chrome device, you will be asked if you wish to opt-out of anonymous data collection. If you opt-out, Google will respect that choice and data will not be send back to them. If, however, you opt-in (the default option), data collected from your machine will be anonymized (any personally identifiable information will be removed) and periodically sent back to Google servers.
Specific metrics that are tracked are called "Histograms", and since Chromium OS is an open source project, we have access to the current list at any given time. Please note that again, while this information is collected, it is done anonymously.
You can also view what data your device is sending back to Google by opening a new tab and visiting about:histograms.
You may opt-out of the data collection at any time by disabling the option in Menu -> Settings -> Show Advanced Settings -> Automatically send usage statistics and crash reports to Google.
Google does not sell your information to advertisers, that is a common myth and is entirely inaccurate.
Instead, an advertiser joins Google's AdWord program and fills out the basic demographic information they would like to target. A sample profile may be along the lines of "someone who is interested in cats, is using a desktop computer, and lives in the United States". Google then uses what they know about their users (from their search history, for example) to match that advertisement to users that are likely to find it useful. At no point in time does the advertiser have access to any of your personal information. They quite simply don't know who you are, Google handles that middle process.
Opting out of Google's Targeted Advertising can be done permanently by installing a Chrome Extension from Google that persistently stores your opt-out preference across user sessions.
No, Google employees do not read your email. Yes, Google computers do scan your email for specific keywords to detect spam, serve relevant advertising, and comply with the law.
It's worth noting that keyword scanning is considered an industry-wide practice, and is not malicious or rare. It is overwhelming common among ad-supported email providers.