In SharePoint 2013 (and SharePoint Online as part of Office 365) you can use metadata navigation for a list of library. Metadata navigation enables users to filter and find items or documents in a list or library by using a navigation tree in the quick launch navigation (the navigation on the left). The navigation also comes with Key filters to filter the list.
It is easy to setup this kind of navigation, just follow these steps:
Before you can use the metadata navigation you need to turn on the metadata navigation and filtering feature via site settings > site features.
In my example I created a document library and added a choice column with 3 values.
To enable the metadata navigation you need to go to the list (or library) settings and click on metadata navigation settings.
In the next screen you select the column you want to use in your navigation. Remember that only field of these types are available: Content Type, Single value choice fields and managed metadata fields.
Below this you can configure the Key Filters:
Remember that you can only use columns that are the following types: Content Type, Choice fields, managed metadata fields, Person of Group fields, Date and time fields and number fields.
After confirming you see that there is a tree view navigation in the quick launch navigation bar (on the left):
By using this navigation you can filter the document library.