What is interface and why we implement interfaces ?

Interface defined by using interface keyword .In visualstudio you can directly add class as a interface its not a class it behaves as a template of the class. Interfaces describe a group of related functionalities that can belong to any class or struct.Interfaces are provided in C# as a replacement of multiple inheritance because C# does not support multiple inheritance and it's very necessary to inherit the behaviour of one or more classes.classes can only implement the methods defined in the interface because in C#, an interface is a built-in keyword that declares a reference type that includes method declarations. if a base class implements an interface, the derived class inherits that implementation.

I am giving an example to better understand :-
Interface which contains profile name strings
public interface IProfile
{
string FirstName {get;}
string LastName {get;}
}
Implements IloggedUser interface.
public interface ILoggedUser
{
IProfile Profile
{
get;
set;
}
}
Class LoggedUser which implements ILoggedUser interface.
public class LoggedUser : ILoggedUser
{
protected IProfile _profile = null;
Constructor
public LoggedUser(IProfile Profile)
{
_profile=Profile;
}

public static LoggedUser Create(IProfile Profile)
{
LoggedUser User = new LoggedUser(Profile);
return User;
}
public IProfile Profile
{
get
{
if (_profile== null)
{
return _profile;
}
}
set
{
_profile=value;
}
}
}
Class profile which implements IProfile interface.
public class Profile : IProfile
{
string _firstName = string.Empty;
string _lastName = string.Empty;
public string FirstName
{
get
{
return _firstName;
}
}
public string LastName
{
get
{
return _lastName;
}
}
}