Theoretically speaking, themes dictate a WordPress website’s design and presentation, and plugins dictate its functionality. This is certainly the intent as outlined over at WordPress.org: …the WordPress Theme system is a way to “skin” your weblog, [and] plugins are tools to extend [its] functionality…
There are a lot of factors that influence performance on your WordPress site, and one of the tricks that “experts” will often tell you to do is avoid plugins. They will tell you it is better to place functionality inside of your theme, instead of activating a plugin. Is this true…
A common, yet unfortunate practice in the WordPress community involves filling theme functions.php files with tweaks and functionality that is key to a site. The reason this is a bad idea, in short, is that it will tie your critical site functionality to a theme that will eventually change. Good news, though: there is a much better, smarter alternative. It’s called a functionality plugin…