The newest top of the line TVs are smart TVs. Smart TVs are TVs that work out-of-the-box and offer smart functionalities, such as applications allowing you to access online services like YouTube, Spotify or Netflix, without requiring any extra action. This is because Smart TVs have an integrated processing unit (which can be thought of as a small-scale computer) that provides the OS, smart applications, integrations, and communication, and handles TV reception, decoding and display.
If you have an older TV, which does not have such a unit, you do not necessarily have to replace it with a newer model. If it still works, you can buy a tiny extension module, which you plug into the TV with an HDMI cable, just like a DVD player. This device has its own remote control, its own OS, applications and potentially, depending on its manufacturer, its own app store. This way, you can turn a ‘standard’ TV into a smart one.