GRealm is a mod to Terraria which adds a variety of content, including new Enemies, Items, Blocks, and more! This mod is spread across all vanilla tiers (and in some cases, even creates tiers of it's own), and can be found throughout an entire playthrough. Content includes an original event, Bosses, many new items, and a unique cast of foes to challenge you!
Base Mod is an API of sorts that adds many internal functions to ease the creation of mods. It's not only for GRealm, however; Any mod maker with the knowledge is invited to use it for their own mods.