Ranking Functions

Following are the Ranking functions in SQL Server 2008 R2;

RANK

Returns the rank of each row within the partition of a result set. The rank of a row is one plus the number of ranks that come before the row in question
If two or more rows tie for a rank, each tied rows receives the same rank. For example, if the two top salespeople have the same SalesYTD value, they are both ranked one. The salesperson with the next highest SalesYTD is ranked number three, because there are two rows that are ranked higher. Therefore, the RANK function does not always return consecutive integers.
The sort order that is used for the whole query determines the order in which the rows appear in a result set.

DENSE_RANK

Returns the rank of rows within the partition of a result set, without any gaps in the ranking. The rank of a row is one plus the number of distinct ranks that come before the row in question.
If two or more rows tie for a rank in the same partition, each tied rows receives the same rank. For example, if the two top salespeople have the same SalesYTD value, they are both ranked one. The salesperson with the next highest SalesYTD is ranked number two. This is one more than the number of distinct rows that come before this row. Therefore, the numbers returned by the DENSE_RANK function do not have gaps and always have consecutive ranks.
The sort order used for the whole query determines the order in which the rows appear in a result. This implies that a row ranked number one does not have to be the first row in the partition.

NTILE

Distributes the rows in an ordered partition into a specified number of groups. The groups are numbered, starting 
at one. For each row, NTILE returns the number of the group to which the row belongs.
If the number of rows in a partition is not divisible by integer_expression, this will cause groups of two sizes that differ by one member. Larger groups come before smaller groups in the order specified by the OVER clause. For example if the total number of rows is 53 and the number of groups is five, the first three groups will have 11 rows and the two remaining groups will have 10 rows each. If on the other hand the total number of rows is divisible by the number of groups, the rows will be evenly distributed among the groups. For example, if the total number of rows is 50, and there are five groups, each bucket will contain 10 rows.

ROWNUMBER

Returns the sequential number of a row within a partition of a result set, starting at 1 for the first row in each partition.

The ORDER BY clause determines the sequence in which the rows are assigned their unique ROW_NUMBER within a specified partition.