The title is self-explanatory, it's an inline heater that you connect to a canister. I was skeptical at first before ordering it because it's relatively new and there isn't a ton of people using it. I got mine last week and it's awesome. First of all, you no longer have an ugly glass tube sticking in your tank and you don't have to worry anymore about your p's breaking it. Since it is plugged in the return hose of your filter, the heated water is evenly distributed in the tank ! It is pretty easy to set up and very accurate. It's a bit pricier than a standard heater but just for the fact that it is no longer visible in your tank, it is totally worth it. I think we need a product review for this wonderful heater !