What is a plugin / theme?
The WordPress platform basically makes use of three pillars, which are WordPress Core (the WordPress software itself), Plugins and Themes.
WordPress Core is managed by the WordPress team itself and provides the framework for a WordPress based site.
A WordPress Plugin is a custom written software that extends the functionality of WordPress Core. Famous WordPress plugins like the WooCommerce e-commerce solution for example, allow you to create your own online shop right within WordPress.
A theme on the other hand is not necessarily meant to extend the functionality of a WordPress site, but defines the look of it. This is why a WordPress website is not immediately recognizable as a WordPress website, but can look like no other website on the internet.
Why do you need a custom plugin / theme?
Whilst the WordPress plugin and theme databases hold numerous free plugins and designs, a custom design or functionality is often required. I can help you with the modification of your existing design, or turn an existing website into a WordPress site.
Do you need a special functionality for your website, like for example add a user to an internal database when he sends you a contact message? This is exactly what plugins are meant for. Or, do you need to make your website accessible via mobile phone? A custom theme allows you to present your companies website beautifully, to mobile users.