Windows 7 is a version of Windows that succeeded Windows Vista. Compared to its predecessor, it provides high speeds and streamlines the interface while maintaining valuable features such as the User Account Control sandbox and the Aero graphical enhancements. In many respects, it is very moderate sitting at the center of a web of operating systems with many different traits.
The biggest changes to Windows 7 aren't as obvious as booting up to the desktop (though the background and default colors differ from Vista!). 7 reordered major interface elements and applets from the Control Panel and beyond. Enough elements more (such as the Start Menu) closely resemble its predecessors, though, that it's still fairly familiar for anyone moving to it and overall its changes are positive and efficient. It also runs very smoothly compared to XP and Vista, and its taskbar and other elements use very slightly less screen real estate.
Overall, Windows 7 is a great choice of operating system for just about anyone. It is a rock-solid operating system with few flaws, save that support for it will end sooner than other operating system options. If you are willing to learn how to navigate the 7's Control Panel and get used to a few other unique traits, Windows 7 is an all-around fantastic choice.