Just a day ago, while I was surfing Wikipedia about SQL Server, I came across this generic architecture image. I found it interesting. Click on image to view it in large size.
The physical structure of the database is divided into the MDF and LDF. The part of MDF contains file group, data files, tables and indexes, extended and page. The LDF file contains a transaction log file. The physical architecture is about how the data is actually stored in the file system. Page, extend, database files are physical architecture.
The logical architecture is about how the data are logically grouped and presented to the end user. It contains tables, constraint, stored procedure, function and other logical architecture.
The page is the smallest unit of stored in SQL Server with the size of 8 KB. There are two kinds of extended and each of the extended contains 8 adjacent pages. There are two types of extended uniform extended and mixed extend. The new table or index is allocated pages usually form mixed extents. Once the table or index overgrows the size of 64 KB or 8 pages, it will change its storage from mixed extended to uniform extent.