ACID (an acronym for Atomicity Consistency Isolation Durability) is a concept that Database Professionals generally look for when evaluating databases and application architectures. For a reliable database all this four attributes should be achieved.
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.