Ok I see what your saying if I draw a hud on the 15" or 12" mac book vs a big monitor it will be tiny.
Isnt this the case with everything though there are some 1080p phones so it should be standard to attempt to get the screen size/dpi from the os no matter what display so is retina just the marketing term for rendering to a scale that fits your monitor?
Yeah, this is an issue with mobile devices, but iOS and Android thought about this from the beginning and provide ways for the programmer to handle this. For example, on Android, you can provide multiple versions of images (low, medium, high, and extremely high DPI). You can also provide measurements in inches/centimeters or device independent pixels (these aren't actual screen pixels, they're kinda like points), or if you don't care about properly handling screens with different DPIs, you can use raw pixel measurements (but this is a very bad idea). Android also lets you use different layouts depending on the screen size and resolution, and if you, the developer, provide different layouts for different screen sizes or resolutions, it will automatically pick the layout that most closely matches the device. Since iOS and Android have been able to do this from the get-go, developers have been able to nicely handle different screen sizes and DPIs. However, desktop operating systems were invented long before high DPI screens ever came around, and it's only now that desktop operating systems are really trying to provide tools and APIs the developers need to properly handle both low and high DPI screens.
As for what "Retina" is: yeah, it's just a marketing term. The idea is that a "Retina display" has pixels that are so small that from the average viewing distance, they are too small to see individually and your eye can't distinguish one pixel from its neighbor, thus allowing a very sharp image. On the iPhone 5, the Retina display has about 320 DPI; on the 15" MacBook Pro, it's about 220 DPI, and on the 13" MacBook Pro, it's about 226 DPI. Personally, I hate the term "Retina display." It's just Apple's marketing way of saying "This is a high DPI screen." There are other screens out there not made by Apple that have just as high or higher DPIs, but since they're not Apple products don't have the "Retina" marketing term applied.