There are many different types. A recent study compared four of the most popular wristband style wearable devices currently on the market and the top two for accuracy, price and satisfaction were Withings Pulse and Fitbit flex. However, the key to any type is to keep track and try to improve daily to increase activity.
For more info on Fitbit trackers check this out: https://theconversation.com/five-things-you-need-to-know-about-fitness-trackers-70046.