Transact SQL :: HOW CTE Works In Server

i like to know how CTE works in sql server. i need to understand the flow of CTE. here i am pasting one example of CTE which i need to understand how works

DECLARE @StartTime DATETIME, @EndTime DATETIME
SELECT @StartTime = '09:00:00'
SELECT @EndTime = '17:30:00'
;WITH interval_cte(StartTime, EndTime) AS

[code]....

1) when CTE is declared then why fields name is used ;WITH interval_cte(StartTime, EndTime) AS ? what is the use of these field name.i do not understand how this line works

SELECT EndTime, DATEADD(mi, 30, EndTime) FROM interval_cte
WHERE EndTime < @EndTime

it has no alias define DATEADD(mi, 30, EndTime) ?how end time is increasing in loop ?which line increase end time value ?