Similar

Both are free and open source web applications written in PHP which use a database such as MySQL.
Both can run on standard, inexpensive hosting.
Both use jQuery

Different

Tiki is more centralized. Tiki is the all-in-one model while Foo (like Joomla! and Drupal) is the small-core-and-add-what-you-need model. Each model has its pros & cons. Because of these design choices, Tiki is the CMS with the most built-in features. With Foo, you can ...
- Tiki has all the features built-in (and you just activate/deactivate features), thus, every Tiki instance of a given version has the same code base. This makes it easier for a hosting company and for upgrades. In contrast, if you maintain dozens of Foo sites, they will have different modules/plugins installed depending the use case.
- When Tiki is upgraded, all the features are supported and the upgrade is smooth. In Foo, some of your plugins/extensions may have become abandoned or be incompatible with the new version.
- In Tiki, forums and social networking are done with built-in (optional) features, whereas in Foo, they are ...
While Tiki is a very large and popular project (over 250 contributors to main code base and over 900 000 downloads), Foo has a ...
Tiki is Wiki-centric, whilst Foo is .... Please see (if) Why Wiki Syntax Is Important to your project.
The Tiki Community has a Dogfood philosophy while MediaWiki is used for the Foo wiki.
Foo is GPL, Tiki is LGPL. The main difference between the GPL and the LGPL is that the latter can be linked to (in the case of a library, 'used by') a non-(L)GPLed program, which may be free software or proprietary software.
Differences in themeing
- Tiki uses the Smarty template engine (which has many benefits), and by default, Smarty security settings prevent the use of PHP in Tiki templates, but you can override this if you need.
- Because of use of PHP in templates, Foo themes actually come with added functionality. Thus, if you change themes, you can lose some functionality. In Tiki, switching themes never makes you lose functionality and only affects the presentation (colors, layout).