In my next few posts I’m going to write about building classification schemes. So it’s probably a good idea to explain the definitions I use upfront.
Before you take me to task, bear in mind that my context is the information governance space, and while I make reference to contemporary eCM systems and information governance standards, I draw widely from other disciplines.

Because there are so many definitions to choose from so many similar disciplines, I’ve endeavored to build a glossary of information governance terms which provides both context and source.

Metadata
Structured, machine-processible information that describes and/or enables finding, managing, controlling, understanding or preserving other information over time. Source: AGLS Metadata Standard Part 1

Taxonomy
In general terms, systematic classification according to principles or general laws. Source: Dublin Core Metadata Initiative

I find the following quote useful when explaining the difference:
At its simplest, a taxonomy organizes information, and metadata describes it. For the taxonomy to be able to organize the information, terms need to be stored as metadata. It all works together to make the content findable, recognizable, and useful. http://blog.braintraffic.com/2012/03/an-intro-to-metadata-and-taxonomies

Information architecture
The blueprint that describes how information is organized and structured. Or;
The set of rules that determine what, and how and where, information will be collected, stored, processed, transmitted, presented, and used. businessdictionary.com

File plan
A pre-determined logical and systematic structure into which records are arranged and intellectually stored according to subject groups and subjects to facilitate efficient retrieval and disposal of records. National Archives and Records Service of South Africa

Faceted classification scheme
A faceted classification system allows the assignment of an object to multiple taxonomies (sets of attributes), enabling the classification to be ordered in multiple ways, rather than in a single, predetermined, taxonomic order. Wikipedia Extract.