Hello and thank you for using Just Answer. My name is XXXXX XXXXX I will be happy to assist you.
Generally the word 'mock up' is used in these terms.
For example say you have not yet started laying out the website in code or whatever platform you are using BUT you have laid out the overall design in photoshop , this would be a mock up.
Another term would be wireframe. A wire frame is is a visual guide that represents the skeletal framework of a website. It can be used to show the overall layout as well as the structure of the site such as the navigation menu's.
I have never used the term skeleton but I believe it is interchangeable with wireframe, they are basically visual representations of the site structure. Here is a picture of a wireframe:
So in simple terms, a mock up shows how the site potentially would look when finished, this means colors, layout , everything.
The wireframe/skeleton shows the structure of the website without the design elements such as colors, fonts, and things of that nature.
Platform in computer context can mean many things. For example one of the most common ways it is used is for operating systems. For example Windows is a platform, mac osx is a platform.
It can also be used for programming languages like flash or java.
If you need further help please do not hesitate to reply back and let me know.