The simplest way to add mods to your Minecraft game is through a tool called Minecraft Forge. Because the modding community is so large, modders have you covered. It takes a little bit of know-how but is really very easy. Once you've picked what kind of mod you want, it's time to add it to the game. But mods have provided deeper, more comprehensive gameplay since the early years of Minecraft. They change the core experience of the game and let you fine-tune your experience. A lot has been added to the game's core (or 'vanilla') experience in the past decade.
Mods, for those of you who don't know, are downloadable content made by fans of Minecraft that MODify the game (get it?). A big part of the reason for that is how easy the game is to modify, or mod. Yet, it still has a strong following even from those who played it when it first came out. The game has been out for 10 years after all. But even with all of this freedom, Minecraft can still get stale after a while. You can live in a castle, or an entire town, or a scale model of New York City if you want to. If you want to play a straightforward survival game, that's there. You can play in a sandbox world that's entirely your own, or do it with friends. Minecraft is one of the most popular games in the world for a reason.