Google is a large American internet company. It started as a search engine when it was founded in 1998, and has since made massive inroads with online advertising technology, cloud computing, and software. Google has a few official apps for Windows Phone and Windows, but has infamously avoiding developing for Microsoft's mobile platforms.
Google makes a few products run on Windows. On Windows Phone and Windows 8.1 there's an official Google Search app. They also make the popular web browser Chrome for the Windows desktop. Outside of those apps, Google has yet to bring official apps for their products like YouTube, Google+ and Gmail to Windows.
Don't expect to see any official Google apps on Windows with the Mountain View company putting more development effort into Chrome OS (their desktop operating system) and Android (their mobile operating system)