Please look at the documantation that provided by Micorosoft SQL 2005

TABLESAMPLE (10 PERCENT) /*Return a sample 10 percent of the rows of the result set. */

TABLESAMPLE (15 ROWS) /* Return a sample of 15 rows from the result set. */.

SYSTEM specifies an ANSI SQL implementation-dependent sampling method. Specifying SYSTEM is optional, but it is the only sampling method available in Microsoft SQL Server 2005 and is applied by default.

TABLESAMPLE SYSTEM returns an approximate percentage of rows. It generates a random value for each physical 8 KB page in the table. Based on the random value for a page and the percentage specified in the query, a page is included in the sample or excluded. Each page that is included returns all rows in the sample result set. For example, when specifying TABLESAMPLE SYSTEM 10 PERCENT, SQL Server returns all the rows on approximately 10 percent of the specified table's data pages. If the rows are evenly distributed on the pages of the table, and if there is a sufficient number of pages in the table, the number of rows returned should approximate the sample size requested. However, as the random value generated for each page is independent of the values generated for any other page, it is possible that a larger, or smaller, percentage of pages than requested are returned. The TOP(n) operator can be used to limit the number of rows to a given maximum