Secret coins are a feature added in update 1.6. They were hidden in every main level in Geometry Dash, and some are awarded from map packs. Despite the fact that they seem like an extra in the game, they are required to unlock Clubstep, Theory of Everything 2, and Deadlocked. They only appear in the Robtop levels, not user created levels. More on Robtop levels here.

Stars were an extra feature added in update 1.3. They are earned from completing verified levels. They help to determine placement on the leaderboard along with helping to progress paths and earning achievements.

User Coins are a collectable added in update 2.0. They contribute to unlocking icons and achievements and are the equivalent to secret coins in user levels.They can only be collected from rated levels. They're also used to earn achievements.

Mana Orbs were a featured added in update 2.1. They are a currency earned by playing levels that are used as a currency in most shops. A mathematic formula determines the amount of orbs earned based on difficulty. Orbs are used at all but the diamond shop to purchase different things. More on shops here.

Diamonds are a collectable added in update 2.1. They can be used to purchase items from The Diamond Shopkeeper and as a currency in the icon editor. You can earn Diamonds from quests and completing daily levels and weekly demons.

Keys are a collectable added in update 2.1. They can be used to open up chests in The treasure Room. One is given after earning 500 orbs from regular gameplay. there are 1 key chests, 5 key chests, 10 key chests, 25 key chests, 50 key chests, and 100 key chests. More on the treasure room and chests here.