PInvoke
Platform Invocation Services (PInvoke) allows managed code to call unmanaged functions that are implemented in a DLL.
This tutorial shows you what you need to do to be able to call unmanaged DLL functions from C#. The attributes discussed in the tutorial allow you to call these functions and have data types be marshaled correctly.
Visit MSDN article link to get whole story about how to use unmanaged dll in C#.
Marshal Class
Provides a collection of methods for allocating unmanaged memory, copying unmanaged memory blocks, and converting managed to unmanaged types, as well as other miscellaneous methods used when interacting with unmanaged code.
MarshalAs
There is default marshaling from .Net to handle the interop from managed code (c#) to unmanaged code. Using MarshalAs gives you a chance to change the unmanaged type to marshal with.
from MSDN:
"Default Marshaling and Specifying Custom Marshaling for Parameters to Unmanaged Methods
When calling an unmanaged function from C# code, the common language runtime must marshal the parameters and return values.
For every .NET Framework type there is a default unmanaged type, which the common language runtime will use to marshal data across a managed to unmanaged function call. For example, the default marshaling for C# string values is to the type LPTSTR (pointer to TCHAR char buffer). You can override the default marshaling using the MarshalAs attribute in the C# declaration of the unmanaged function.
The MarshalAs attribute can be placed on method parameters, method return values, and fields of structs and classes. "