"SQL Server memory is primarily used to store data (buffer) and query plans (cache). In this article I'll show how much memory is allocated to the data buffer (or RAM). I'll explain how to determine what tables and indexes are in the buffer memory of your server.
SQL Server stores its data in 8KB data pages. As these pages are read off disk they are stored in memory. This is referred to as buffer memory. A list of all the data pages in memory is stored in the dynamic management view sys.dm_os_buffer_descriptors."