If you use the web, it's important to realize that Google partners with many different websites and stores information about your interests and demographics, even if you don't have a Google account. All of this information is accessible from the Google Privacy Center, which contains many useful privacy tools.
For example, if you perform a Google search for dogs, Google might share this information with YouTube. Then if you were to search YouTube for pets, videos about dogs would appear higher in the search results. This may sound like a lot of personal information to share with one company, but it's important to remember that Google only uses this information to improve your search results and other Google services.
Google offers a comprehensive range of web services, but there are alternatives that offer similar services (we've listed some below). While comparable in many ways, these other sites cannot offer the convenience of a single Google account. Still, you may want to consider whether you feel comfortable sharing all of your information and online behavior with Google.