The most basic trackers are glorified step counters (aka pedometers). Pedometers have been around for some time and can be found in pretty much any drugstore. A schmancy device in this category is the Fitbit Zip ($50), which syncs to an app on your phone, but you can spend as little as $15 for a clip-on device like the Omron HJ-320.
Some basic trackers can measure sleep quality too.
This includes the Misfit Flash ($20), Jawbone UP Move ($35), Fitbit One ($90), and Fitbit Flex ($80). These devices typically have integrated lights that can show you your progress, instead of a display that can show information like the time (except for the One, which has a small display).
The next tier are basic trackers with "smart" features for text and call alerts.
Fitbit's Alta ($130) and the Misfit Ray ($100) have vibration alerts for calls, texts, and alarms. You can also use the Ray to take selfies and control music.
More advanced fitness wearables have basic activity tracking and heart-rate monitoring.
The Jawbone UP3 ($93), Charge HR ($150), Garmin Vivosmart HR ($150), Basis Peak ($195), and the Adidas Fit Smart ($140).
Most smartwatches have fitness features built in.
The Moto 360 ($300), Apple Watch ($300 to $350 for sport model), Microsoft Band ($175), and Huawei Watch ($350) are devices with full-color touchscreen displays, heart-rate sensors, and activity tracking capabilities.
Finally, there are running watches with GPS tracking included.
These devices use satellites to track your routes, pace, distance, and elevation. The Fitbit Surge ($250), Garmin vivoactive HR GPS ($300) (waterproof! good for swimmers and triathletes!), Polar M400 ($158) are good for more serious athletes who are looking to track their mileage with a high degree of accuracy, without their phones.