Lure modules attract Pokémon for 30 minutes, and everyone who is near the PokéStop will benefit (unlike incense, which only attracts Pokémon to one person).

Lure modules can be found at pokéstops or bought using pokécoins from the store. Lure modules currently cost 100 Pokécoins for one module, or 680 Pokécoins for six. They're also distributed as one of the rewards handed out when the player reaches various levels.

Here's a quick explainer video: