What is CTE ?

Create Table tblCategories
(
CategoryID Int Constraint PK_tblCategories_CategoryID Primary Key,
CategoryName VarChar(100),
ParentCategoryID Int Constraint FK_tblCategories_ParentCategoryID References tblCategories(CategoryID)
)
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Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(1,'Languages',Null)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(2,'Networking',Null)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(3,'Databases',Null)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(4,'Visual Basic',1)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(5,'C#',1)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(6,'Java',1)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(7,'VB.Net',4)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(8,'VB 6.0',4)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(9,'Desktop Application Development with VB.Net',7)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(10,'Web Application Development with VB.Net',7)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(11,'ActiveX Objects and VB 6.0',8)
Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(12,'Network Security',2)
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Select * From tblCategories Where CategoryID = 1
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With cteCategories
AS (
Select CategoryID,CategoryName,ParentCategoryID
From tblCategories
Where CategoryID=1
Union All
Select C.CategoryID,C.CategoryName,C.ParentCategoryID
From tblCategories As C Inner Join cteCategories As P On C.ParentCategoryID = P.CategoryID)
Select CategoryID,CategoryName,ParentCategoryID From cteCategories