Lures Modules are items you can use to enhance PokéStops. Installing a Lure will attract wild Pokémon to a PokéStop. Under normal circumstances, Lures will last 30 minutes once installed. The effect will benefit you and other Trainers nearby. Lures are available in the Shop and may also be obtainable from Research tasks and leveling up. To use a Lure, tap on a PokéStop and select the module slot below the PokéStop description. Then, choose the type of Lure Module you’d like to install.
A standard Lure Module that attracts Pokémon to a PokéStop.
Glacial Lure Module
Glacial Lure Modules attract certain Pokémon that love the cold, such as some Water- and Ice-type Pokémon.
Mossy Lure Module
Mossy Lure Modules attract certain Pokémon that play in the leaves, such as some Bug-, Grass-, and Poison-type Pokémon.
Magnetic Lure Module
Magnetic Lure Modules attract certain Pokémon that are drawn to electromagnetic forces, such as some Electric-, Steel-, and Rock-type Pokémon.
Some Lures can also enable certain Pokémon to evolve in ways they otherwise wouldn’t. If you’re standing near a PokéStop with one of these installed, you’ll notice a special option to evolve these Pokémon on the Pokémon’s detail page.
Source: Official Pokémon GO support page