Terraria is a 2D sandbox game. Now before you say its a 2D Minecraft copy:
1) Its has 12 different bosses, that's 10 more than Minecraft
2) This game is DIFFICULT, unlike Minecraft. (except if you play the X-Box or IOS version)
3) This game has lot more content than Minecraft
Now that I have those points across, I will tell you about some of the gameplay. The gameplay consists mostly of exploring than combat, because even though the world isn't massive there are a lot of things to find and explore, find a spider cave? EXPLORE IT! find a sky island? EXPLORE IT! find the underworld? EXPLORE IT! (yes, you can got to hell in this game, its kind of like the nether for Minecraft). This games combat is like Minecraft, you chose which direction your sword will swing, and then you just rapid click a lot. READY? AIM! CLICK!!!!!!!!