USE WITH CAUTION - Technical Support for modded Minecraft crashes and issues are off-topic. This tag is for questions relating to the content, features or gameplay of mods in Minecraft.
Minecraft is one of the games with the biggest modding community, despite not officially offering a modding API. Fan-made mod frameworks like Forge, Bukkit or LiteLoader add this functionality and are used by mods that add more technical or magical elements or even entirely new dimensions with their own creatures and resources. Some mods just improve little details, like fixing bugs, or add compatibility with outside resources, like ViveCraft for VR goggles, ReplayMod for cinematic video creation or various 3D printing exporters.
Please also use minecraft-java-edition in addition to this tag. Mods for Bedrock edition and Legacy console edition currently do not exist. Behavior packs are not mods. Datapacks that heavily modify the game's effective behaviour are sometimes called "Vanilla mods", but are not actually mods.
USE WITH CAUTION
Technical Support for modded Minecraft crashes and issues are off-topic as per this meta discussion.