Curse is an enchantment type, not a creature type (or any other kind of subtype).

Each of the Curses can be attached to any player, including the player who cast the Curse.

The Curse subtype has no inherent rules meaning, but two cards in the Innistrad set (Bitterheart Witch and Witchbane Orb) refer to Curses.

Curse of Inertia’s ability will trigger only once per attacking player per combat phase, no matter how many creatures that player attacks with.

A Curse spell targets the player it will enchant like any other Aura spell, and a Curse stays on the battlefield like any other Aura. If the enchanted player gains protection from the Curse’s color (or any other characteristic the Curse has), the Curse will be put into its owner’s graveyard.

Even though the attacking player chooses the target of the ability, the ability is still controlled by the player who controls Curse of Inertia. Notably, a creature with hexproof controlled by the player who controls Curse of Inertia could be chosen, even though another player may be choosing the target.

The ability won’t trigger when a creature attacks a planeswalker controlled by the enchanted player.