Anything newer than Windows XP will cache often used files/programs in RAM in order to more quickly access them. I know in Windows 8.1, it's broken down in Task Manager that despite being "used" it's still free memory, since Windows can just overwrite it as needed when applications use that amount of memory. Generally, that's what leads to looking like you constantly have most of your RAM in use, most of it's just being used to hold stuff you could use, rather than stuff you're actively using.