Really depends on what type of mod.
Standalone Mods are mods that basically replace or add to base Terraria without the use of a mod loader, the mod replaces the vanilla
Mod Loader Mods are mods that require a mod loader, which the mod loader is a Standalone Mod. Mod Loader Mods are more expansive since you can load mods into the mod loader.
tModLoader is a standalone mod that is a mod loader. This is required for most mods since they use tModLoader's hooks to implement their code. To get mods, you can either download them manually and install by moving the .mod file to "C:\Users\Your Name\Documents\My Games\Terraria\ModLoader\Mods" or through the Mod Browser built into tModLoader.