External blowers are situated away from the ventilation system and tend run very quietly. Units may sit on your roof or external walls and windows. They are often used by professional chefs because of their power and location outside of the kitchen.
Internal blowers are built into the ventilation system. These units are very common among American households because of their efficiency and ease of installation.
Inline blowers are similar to external blowers. The key difference is where the blower is located. An inline blower is located away from the kitchen but inside the home. They are easier to install and don't interupt the external appearance of your home.