AdventureWorks is a Sample Database shipped with SQL Server and it can be downloaded from CodePlex site. AdventureWorks has replaced Northwind and Pubs from the sample database in SQL Server 2005.
The Microsoft team keeps updating the sample database as they release new versions.
For SQL Server 2012 Samples AdventureWorks Database is released at below link:
You can download either of the datafile and create database using the same.
Here is the script which demonstrates how to create sample database in SQL Server 2012.
CREATE DATABASE AdventureWorks2012
ON (FILENAME = 'D:\AdventureWorks2012_Data.mdf')
FOR ATTACH_REBUILD_LOG ;
Please specify your filepath in the filename variable.