TSQL to concatenate column values into a single string

DECLARE @cols AS varchar(3000)
;
WITH YearsCTE
AS
(SELECT DISTINCT YEAR(OrderDate) as [Year] FROM Sales.SalesOrderHeader)

SELECT @cols = ISNULL(@cols + ',[', '[') + CAST([YEAR] AS varchar(4)) + ']'
FROM YearsCTE
ORDER BY [YEAR]

Note the use of ISNULL. This adds a additional "," to separate records. Note the isnull helps to add the additonal "," only to records numbered 2 or greater.