Store Procedure In Cutomer Profile Provider

In customer profile provider, I got the one of  store procedures  as below
question is that : I dont understand the character "N" and "case" used for??

 help...1 ALTER PROCEDURE [dbo].[Profiles_Sel]
2 (
3 @userName nvarchar(50)=null,
4 @inactivityDate datetime=null,
5 @isAnonymous bit
6 )
7 AS
8 BEGIN
9
10 Declare @select nvarchar(2000)
11 Declare @where nvarchar(255)
12 Declare @params nvarchar(255)
13
14 /*Select profiles that meet the specified criteria.*/
15 Set @select=N'Select '
16
17 Set @select=@select+
18 N'ProfileID,
19 UserName,
20 ApplicationName,
21 IsAnonymous,
22 LastActivity,
23 LastUpdated
24 From Profiles
25 '
26
27 Set @where=N'';
28 If(@userName Is Not Null)
29 Begin
30 Set @where=@where+Case When @where='' Then N' Where ' Else N' And ' End + N'(Profiles.UserName Like @_userName+''%'')';
31 End
32 If(@inactivityDate Is Not Null)
33 Begin
34 Set @where=@where+Case When @where='' Then N' Where ' Else N' And ' End + N'(Profiles.LastActivity <= @_inactivityDate)';
35 End
36 If(@isAnonymous=1)
37 Begin
38 Set @where=@where+Case When @where='' Then N' Where ' Else N' And ' End + N'(Profiles.IsAnonymous = @_isAnonymous)';
39 End
40
41 Set @select=@select+@where;
42
43 Set @params=N'@_userName nvarchar(50),@_inactivityDate nvarchar(50),@_isAnonymous bit';
44
45 Execute sp_executesql @select,@params,@_userName=@userName,@_inactivityDate=@inactivityDate,@_isAnonymous=@isAnonymous;
46
47 END
48