I need to write a simple app which will work in a cloud where I can store data about clients and it will be avaiable from Windows and Android and I assume it has to be a website.
What is the difference between ASP .NET Web Forms Applcation and ASP .NET Web Forms Site? Which one should I use?
Web Form Site is nothing more than previously called Website projects where you work with webpages which is just small php like (with Razor/aspx) script files which render as html.
Webform is more like a component oriented framework. Web Form Site or WebSite should only be used for small projects or if the project needs to be compile in C#/VB both language. Also if you want to put your all source code in web-server then it can be used. On contrary with WebForm application you don't need to put code base files to run the projects - you'd be better off only putting scripting .aspx files.
This has been explained in detail in the following question: ASP.NET Web Site or ASP.NET Web Application?
Also, you can check out the differences between the two here: MSDN
Regarding your first question if your app is native or hybrid then you need REST(WebApi in .net world) which any app can access natively regardless of the device and will meet your needs of the "cloud".
If you what you want is a website instead that is "viewable" by mobile devices you may want to make your website "responsive" and you can look at various bootstraps templates that can help you with that.
Regarding your second question I think it has no bearing nor it will make any difference in creating a mobile app which project type you use.
