Currency is used to buy items in-game.
Gigas are used for armor, skills, shields, arm parts, etc.
Coins are a special type of currency used to buy wings, flags, coin minibots, coin heads, coin mercenaries, coin guns, coin trans packs, II skills, coin field packs (Rebirth pack), stashes, combination coupons, and transformation coupons. Coin heads and minibots can be worn with regular giga ones. (You cannot see the regular giga head or minibot while wearing a coin one, but they both still add stats).
Gigas and coin items (trial) are dropped.
Bot stract is gained in PvP after defeating a player successfully. At the moment it does not have a use but it may later on.