Group by controls the presentation of the rows, order by controls the presentation of the columns for the results of the SELECT statement.
EX 1 : Group By helps us display any aggregate of any column based on a field what has repeated names.
select Title,SUM(BaseRate) as Rate from dbo.DimEmployee group by Title
EX 2 : Order By clause helps us display any table based on the values present on that particular column.
select FirstName,Title from dbo.DimEmployee order by FirstName
Reference : Dilip Kumar Jena ( https://sqlexplore.wordpress.com )