Row_Number() , Rank() in SQL Server

RANK(): Returns the rank of each row in the result set of partitioned column

select Name,Subject,Marks,
RANK() over(partition by name order by Marks desc)Rank
From ExamResult
order by name,subject

DENSE_RANK() This is same as RANK() function. Only differencec is returns rank with out gaps.

select  Name,Subject,Marks,
DENSE_RANK() over(partition by name order by Marks desc)Rank
From ExamResult
order by name

in RANK() result set screeshot, you can notice that there is gap in Rank(2) for the name Sam and same gap is removed in DENSE_RANK().

NTILE(): Distributes the rows in an ordered partition into a specified number of groups.
It devides the partitioned result set into specified number of groups in an order.

Example for NTILE(2): 

select Name,Subject,Marks,
NTILE(2) over(partition by name order by Marks desc)Quartile
From ExamResult
order by name,subject

Example for NTILE(3):

select Name,Subject,Marks,
NTILE(3) over(partition by name order by Marks desc)Quartile
From ExamResult
order by name,subject

ROW_NUMBER(): Returns the serial number of the row order by specified column.

select Name,Subject,Marks,
ROW_NUMBER() over(order by Name) RowNumber
From ExamResult
order by name,subject