sql server
From msdn:
The page is the fundamental unit of
data storage in SQL Server. An extent
is a collection of eight physically
contiguous pages. Extents help
efficiently manage pages.This section describes the data
structures that are used to manage
pages and extents. Understanding the
architecture of pages and extents is
important for designing and developing
databases that perform efficiently.
So what commands/utilities are available to visualize the data pages on SQL Server ?
Data pages are stored contiguously when the index is created and when the index is rebuilt. Otherwise, SQL Server will attempt to keep the pages in physical order. That not being possible, logical order is attempted. You can get gaps in a table because of other writes that are happening in the database. SQL Server uses B+ trees for indexes - the leaf (data) level of the index contains the data but is also a doubly linked list that contains pointers to the next and previous pages. This makes it less important that the table be 100% physically contiguous.
Think about it like this: leaf level pages are the end of the road for the data search through the B-tree structure of an index. They contain the data defined in the index definition (or in the case of a clustered index, all of the table's data) and a row locator to the data row (in the case of a nonclustered index).
Non-leaf level pages contain the "road map" to the leaf level pages/data by including the key value as well as a pointer to either another non-leaf level page (depending on the B-tree depth and location of the intermediate page) or the resulting leaf level page (index page for a nonclustered index, and data page for a clustered index).
Edit: Here's a good image to put a picture to the idea.
Best Answer
DBCC PAGE
Paul Randal also has lots of stuff about on-disk structures and articles "Anatomy of a (insert storage thing here)" (some overlap here of course)