Windows Defender is the built-in security solution in Windows 10. It provides basic protection against threats. In recent Windows 10 builds, Windows Defender is part of Windows Defender Security Center. This post explains how to reliably disable Windows Defender in Windows 10.
User Account Control, or just UAC is a part of the Windows security system which prevents apps from making unwanted changes on your PC. When some software tries to change system-related parts of the Registry or the file system, Windows 10 shows an UAC confirmation dialog, where the user should confirm if he really wants to make those changes.