I have seen a lot of C# programs that use the [], for example [STAThread] and then the code follows. Another classic example is [DLLImport].
I know what STAThread means but my question is what is the significance of the square brackets, essentially what do they tell the compiler?
It's an attribute. Attributes are a form of metadata that you can attach to various code elements: classes, methods, assemblies etc.
Some attributes have special meaning to the C# compiler, for instance the [Serializable] probably tells the compiler to emit some code that can serialize an instance of the class (I say 'probably' since I do not know the inner workings of the C# compiler).
You can also create your own attributes (by inheriting System.Attribute). Using reflection you could then at run-time extract information from the attributes.
A simple example would be to create an attribute to specify what kind of input field to use in a HTML form when displaying an object's property.
Some links:
