WordPress is always been spreading updates since early days and that is one of the greatness about the WordPress so when you have update which you can see in your Dashboard, you shall confirm whether you have any customization or not. In that case when WordPress updates you may loose your customization.
WordPress plugin is something which plays very important part in your Website functionality, there are native plugins which comes at the time of setup and there are other plugins you installed for functioning WordPress Website. As we all know it is open source.
The WordPress theme is the outer of your WordPress website or blog. Many of the themes provide versions and compatibility to make sure it matches your WordPress setup. That is why WordPress theme developers keep updating their themes and due to that reason it becomes necessary to make all the components updated.