Both games feature being dropped into a randomly generated empty wilderness, and require you to construct a house to (generally) survive the night. Both games have a crafting system (though how it works is different). Both games feature construction (and mining) in a tile based system. In general, both games feature tiered tools and armour, where you use tools from the ‘first tier’ to get tools from the second.

That said, they are also different games. Minecraft’s main focus (until the ‘Adventure Update’ happens, at least) has been on construction, with no real ‘end game’ beyond gathering a whole bunch of diamond. Terraria is more defined, with bosses, “high level” areas, and generally a lot of content for the ‘end game’. Terraria is the least Minecraft Minecraft clone, but there’s definitely some inspiration there.

If anything, Terraria is a 2d Metroidvania combined with Minecraft.