Details about the importing system dll files

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C#

VB

C#4.0

Hello Friends...
i am using .net 3.5 and i got a code in study that is

Public Declare Auto Function SHGetFileInfo Lib "shell32.dll" (ByVal pszPath As String, ByVal dwFileAttributes As Integer, ByRef psfi As SHFILEINFO, ByVal cbFileInfo As Integer, ByVal uFlags As Integer) As IntPtr
    ' declare some constants that SHGetFileInfo requires

    Public Const SHGFI_ICON = &H100
    Public Const SHGFI_SMALLICON = &H1
    Public Const SHGFI_LARGEICON = &H0         ' Large icon
    Public Const SHGFI_USEFILEATTRIBUTES = &H10
    Public Const FILE_ATTRIBUTE_DIRECTORY = &H10
    Public Const FILE_ATTRIBUTE_NORMAL = &H80

    ' define the SHFILEINFO structure
    Structure SHFILEINFO
        Public hIcon As IntPtr
        Public iIcon As Integer
        Public dwAttributes As Integer

        <Runtime.InteropServices.MarshalAs(Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst:=260)> Public szDisplayName As String
        <Runtime.InteropServices.MarshalAs(Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst:=80)> Public szTypeName As String
    End Structure

i want to know about these keyword, functions, variable.
i want to learn about this. can anyone help me out to learn these things
in detail.