Tiki, from a software engineering standpoint, is a monolithic application. We have strong opinions on the advantages of this approach to avoid development fragmentation and to ease refactoring. Please see: http://pluginproblems.com/ and Versions.
Tiki has various mechanisms to allow extensions and customisations. In fact, more ways than most modular software.
If you are new to Tiki, before you start down this list, keep in mind that Tiki is extremely flexible. In most cases where you'd use a plugin or write code in another system, in Tiki you are expected to configure one of the 2000+ preferences, use an existing Wiki plugin or setup a tracker.
In Tiki, the visual theme affects only appearance, not functionality, so a theme can't introduce a security risk.
Using the Look and feel administration panel, you can add snippets of code, which will be stored in the database, and survive upgrades:
Simpler than writing a theme if you have few things to customize.
In Tiki terminology, a "profile" is a set of site configuration preferences and website item creation instructions that are applied as a group. They only add or edit content in the site's database. Some profiles are simple, like a contact form, and some are more complex.
Also called the Composer Web Installer, and introduced in Tiki18, this feature is for the installation and management of external software packages using Composer. Its main reason to exist is to facilitate installation of PHP lib dependencies that the Tiki community can't or prefers not to bundle. Ex.: incompatible licence, too big, too niche, etc. Please note that the code that uses these libraries remains managed in the main code base. To illustrate: The mPDF library is fetched as a Package, but all the other code is in Tiki.
Allows system administrators to set some preferences using ini files in the file system to override the preferences in the Tiki database.
Arbitrary PHP code can override parts of Tiki by dropping it in the Path structure, For instance Custom php file (as of Tiki1😎 _custom/lib/setup/custom.php
Wiki plugins are used to embed features and interactive data and functions in any wiki text area in the Tiki site, including in wiki pages, blogs, articles, forums, and so on. Importantly, although they are called "plugins", they are not third-party or aftermarket additions to Tiki - they are included with the Tiki installation.
Probably poorly named, in Tiki a module is a box of content that can be placed in layout areas (eg: topbar, left, right, bottom, etc) and also in text areas (eg: wiki pages). Other software tend to call them blocks, or boxes. All modules are included with the default Tiki installation.
There was an Addons feature that existed from Tiki 14 to Tiki 19 which was subsequently removed in Tiki 20. If you are writing your own custom code that extends Tiki that you want to install as a Package, you may want to look at Packages that extend Tiki which is available from Tiki 21.