Your cellphone is a double edged sword.
On one hand it is a tool that can be used as a phone, to read your email, look at webpages, for getting directions, etc. On the other hand it is a tracking device that can be used against you and to keep tabs on you.
All smartphones sold today come with built in GPS. You can use it to find your location and figure out how to get where you want to go. It can also be used to track your movements. As of the writing of this post (June, 2016) the average 4G cell phone sold in North America has a GPS accuracy of 3 meters, about 9 feet. This means the cell phone GPS can tell the difference between you standing in front of your car or behind. With a special app installed on your phone your phone can be tracked and, in effect, you are being tracked.
Normally the tracking app has to be installed on your phone which means someone has to physically install the app. Most cell phones in North America are either iPhones, about 28% of the market, or Android which is about 67% http://www.ibtimes.com/apples-ios-still-getting-crushed-android-us-2130868. The rest is either Windows phones of Blackberries. Both iPhone and Android are alike in that their is a central repository for apps to be installed on your phone.