This library is organized by folders, and you will find every model inside its own folder and its own file. Into the file every model is grouped with the same name of the folder so you can find it easily when you load it into your scene.

Blender is able to load materials, objects or any other datablock from an external blend file, and it can do it by Link or Append those materials, objects, etc…

Link or Append

To Link a model you can use the hotkey Ctrl-Alt-O or find the Link function in the File menu. To Append a model you can use the hotkey Shift-F1 or find the function Append in the File menu

 By Linking a model you will load a reference of the original model. You can't change the properties of the linked model in your scene. If you make changes to the original model then all your scenes linking to that model will be updated with the changes the next time you open the file. If you use the Append option you will be able to modify all the properties of the objects (size, rotation, materials, etc..). So you will have a full copy of the data into your .blend.

Models as groups

The objects of every model are grouped with the name of the model. To Append or Link one model of the library you must select the blend file first and then the group that you will find inside the Group folder of the file.

If you are linking the model then it will appear parented to an empty at the 3D cursor position by default. You can use the empty to move, rotate or resize the model. 

 Tip: You can change the size and the shape of the displayed empty in the Data section of the Properties editor. 

 If you are Appending the model then it will appear at the center of the grid as a green group.