Microsoft® SQL Server 2012 Unleashed

Extents

If SQL Server allocated space one page at a time as pages were needed for a table (or an index), SQL Server would be spending a good portion of its time just allocating pages, and the data would likely be scattered noncontiguously throughout the database. Scanning such a table would not be very efficient. For these reasons, pages for each object are grouped together and allocated in extents; an extent consists of eight logically contiguous pages.

When a table or index is created, it is initially allocated a page on a mixed extent. If no mixed extents are available in the database, a new mixed extent is allocated. A mixed extent can be shared by up to eight objects (each page in the extent can be assigned to a different table or index). ...