Questions about the .NET Framework

Hey, I want to ask few questions regarding the .NET Framework related toVisual Studio Express 2013 for Web with Update 3.

1. What's the .NET Framework?

2. What's it's use in developing web applications and websites in Visual Studio Express 2013 for Web with Update 3?

3. The first tutorial on developing a website called 'Wingtip Toys' is using the .NET Framework 4.5.2. I had .NET Framework 4.5.1 earlier but after installingVisual Studio Express 2013 for Web with Update 3 I manually installed .NET Framework 4.5.2 from here. Now, when in the Templates window of the project I don't see .NET Framework 4.5.2 but the NET Framework 4.5.1.. Why is it so?

4. Will it effect my 'Wingtip Toys' project if I use an earlier version which I'm doing so?

5. Writing the C# code given here starting with:

[Display(Name = "Price")]
        public double? UnitPrice { get; set; }

        public int? CategoryID { get; set; }

        public virtual Category Category { get; set; }

I get an error line on :

public virtual Category Category { get; set; }

the second Category saying: 'WingtipToys.Models.Product.Category is a 'property' but is used as 'type'.' Why?

Thanks,

Kvaibhav01.