Pick and Choose

Starting from Minecraft version 1.11.
Resource packs can alter textures, sounds and models. For example change your crosshair or select a new menu background.

Starting from Minecraft version 1.13.
They can run commands in-game, control loot tables, change crafting recipes and alter achievements.

Starting from Minecraft version 1.13.
Crafting Tweaks are a separate category of datapacks. They can change or add crafting recipes and add functionality.