Block Library - DesignCenter

AutoCAD created common blocks and dynamic blocks, store in the library DesignCenter

View tab > Palettes panel > DesignCenter

You can toggle On Off the Tree View, Preview and Description

C:\Program Files\Autodesk\AutoCAD 2015\Sample\en-us\DesignCenter

Double-click on the block
You can change the parameters when insert the block

Blocks created in inches will automatically convert to metric when insert into a metric drawing