Access modifiers
From docs.microsoft.com:
public
The type or member can be accessed by any other code in the same assembly or another assembly that references it.
private
The type or member can only be accessed by code in the same class or struct.
protected
The type or member can only be accessed by code in the same class or struct, or in a derived class.
private protected (added in C# 7.2)
The type or member can only be accessed by code in the same class or struct, or in a derived class from the same assembly, but not from another assembly.
internal
The type or member can be accessed by any code in the same assembly, but not from another assembly.
protected internal
The type or member can be accessed by any code in the same assembly, or by any derived class in another assembly.
When no access modifier is set, a default access modifier is used. So there is always some form of access modifier even if it's not set.
The static modifier on a class means that the class cannot be instantiated, and that all of its members are static. A static member has one version regardless of how many instances of its enclosing type are created.
A static class is basically the same as a non-static class, but there is one difference: a static class cannot be externally instantiated. In other words, you cannot use the new keyword to create a variable of the class type. Because there is no instance variable, you access the members of a static class by using the class name itself.
However, there is a such thing as a static constructor. Any class can have one of these, including static classes. They cannot be called directly & cannot have parameters (other than any type parameters on the class itself). A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced. Looks like this:
static class Foo()
{
static Foo()
{
Bar = "fubar";
}
public static string Bar { get; set; }
}
Static classes are often used as services, you can use them like so:
MyStaticClass.ServiceMethod(...);
Classic mode (the only mode in IIS6 and below) is a mode where IIS only works with ISAPI extensions and ISAPI filters directly. In fact, in this mode, ASP.NET is just an ISAPI extension (aspnet_isapi.dll) and an ISAPI filter (aspnet_filter.dll). IIS just treats ASP.NET as an external plugin implemented in ISAPI and works with it like a black box (and only when it's needs to give out the request to ASP.NET). In this mode, ASP.NET is not much different from PHP or other technologies for IIS.
Integrated mode, on the other hand, is a new mode in IIS7 where IIS pipeline is tightly integrated (i.e. is just the same) as ASP.NET request pipeline. ASP.NET can see every request it wants to and manipulate things along the way. ASP.NET is no longer treated as an external plugin. It's completely blended and integrated in IIS. In this mode, ASP.NET HttpModules basically have nearly as much power as an ISAPI filter would have had and ASP.NET HttpHandlers can have nearly equivalent capability as an ISAPI extension could have. In this mode, ASP.NET is basically a part of IIS.
Best Answer
The system.web section is for configuring IIS 6.0, while the system.webserver version is used to configure IIS 7.0. IIS 7.0 includes a new ASP.NET pipeline and some configuration differences, hence the extra config sections.
However...
If you're running IIS 7.0 in integrated mode only, you shouldn't need to add the handlers to both sections. Adding it to system.web as well is a fallback for IIS 7.0 operating in classic mode, unless I'm mistaken. I've not done extensive testing on this.
See http://msdn.microsoft.com/en-us/library/bb763179.aspx for more information.