Below are definitions (or links to definition pages) for the terms you should know before browsing the site, and definitely need to know before uploading mods. The definitions also tell you how the terms are applied on this site, which is a little different than just defining them.

Platforms:
http://mods.oni2.net/taxonomy-1

Mod types:
http://mods.oni2.net/taxonomy-2

Install methods:
http://mods.oni2.net/taxonomy-3

Drupal terms:
Node - a node is the basic unit of storage in Drupal, and contains a piece of Content

Content - anything that's posted by a user; on the Mod Depot, it comes in three flavors, File, Page, and Story; Page and Story are for making pages and blog entries, respectively, and only the administrator can do that

File - not quite what you think, it could be called "Banana" and still serve the same function; "File" is simply another kind of content in the form of some fields that you fill in; however, you can attach actual files to this type; all users can create this type of Content

Taxonomy - the practice of using Vocabularies to label content nodes

Vocabulary - a set of Terms that can be applied to something; "Mod Type" is a vocabulary, for instance

Term - for example, "Package"; sometimes more than one term can be selected; on this site the only place that is true is when picking the Mod Type; sometimes more than one Type applies, thus you can use Shift or Control(Windows)/Command(Mac) to select multiple Types