Chunks are bits of text or code that can be reused throughout a site. These are primarily html but can also be javascript, css, or text. They are called within a document, snippet, TV, or template like so:
Snippets are bits of PHP code that are used to add additional functionality to a site. These are most often used for menus but are also used for galleries, guestbooks, comments, forms, etc. They are called within a document, TV, chunk, or template like so:
Or, if parameters are needed:
[[snippetname?parameter=value¶meter=value]]
TVs are template variables and are values assigned to a particular document. They can be used to specify anything specific to the current page included a header image, sidebar items, category, author name, etc. These can be called within a document, chunk, or template like so:
Plugins are PHP scripts that can interact with MODx events. These eliminate the need to edit the core (most of the time). Current examples included the Quickedit plugin and all Rich Text Editors (FCK Editor [by default], Xinha, and TinyMCE). These are executed when specific MODx events are called.
Modules are large bits of code that can interact with plugins and all other MODx items. These are mainly used for backends for things like galleries (coming soon) and the new Quickedit function. These are run from within the Manage Modules page.