While there exist quite a few methods to track the location of a cell phone and the person using it, only the law enforcement agencies, other than the service providers themselves, are authorized to do so. Illegal approaches to cell phone tracking do exist, but they are best left untouched for obvious reasons. Global Positioning System (GPS) was developed by the US Department of Defense (DOD) for military purpose. It was made available to civilians only in 1983.
Service providers and law enforcement agencies are legally and morally bound to help you, but only if you have a genuine reason. Now the term 'genuine' in this case is subjective. For parents, keeping a track of the whereabouts of their child is no doubt a genuine reason, but they can't go to the agencies for the same. This is exactly where GPS cell phone tracking apps, which use GPS navigation to locate the device (and its user) come into the picture.
Initially, the use of GPS was restricted to getting directions to go from point A to point B. As time elapsed, several new uses of the same surfaced; tracking, wherein the location of a cell phone is traced by the method of trilateration, being one of them. This process is based on the law that it is possible to determine the location of an object when the distance of the said object from three different points is known.
It is one of the several methods that are used by law enforcement agencies to track people on the basis of their cell phone location. In several jurisdictions, these agencies are even entitled to track a person's cell phone to keep a track of his whereabouts without him knowing, i.e., non-consensual tracking.
Over the course of time, some app developers have come up with apps that use the GPS technology to locate cell phones. Most of these apps work on similar lines; on downloading the app on the handset that is to be tracked, you get to monitor the movement of the person using this handset, on a map on your handset or computer. Unlike in the case of non-consensual tracking, wherein you are kept in dark about the fact that your movement is being monitored, this is a case of consensual tracking where the app explicitly requires your permission to do the needful.
There is no dearth of such cell phone tracking apps. While some of these apps come for free, others come for a stipulated fee. You will find them on the iTunes Store, Google Play, or even the app developer's dedicated website, from where you can download them. Some of these apps are invisible, i.e., they don't have any icon. They are designed to work in the background to ensure that the user doesn't know that he is being tracked.
These mobile tracking apps are useful for people who want to keep a track of the whereabouts of their children and family members. Some of these apps come with a feature called Geofence alerts, wherein you get a message when your child moves out of the demarcated area or goes to places where he is not supposed to. Some of these even provide a list of sex offenders in that area. With these apps, you can even keep a track of your family members during natural disasters. They can also help you track your cell phone in case you lose it or it gets stolen.
Lately, the unethical use of this technology, i.e., non-consensual tracking, has been on rise. This, in turn, has made people oppose this practice citing invasion of privacy. In some jurisdictions, a legal procedure has to be followed in order to track the location of a person by tracking his cell phone. While such laws do help in curbing the unethical practices associated with phone tracking, the onus is on us to make sure that we don't resort to such practices in the first place.
Now that must have been disheartening for you if you expected a spy-film-like method of tracking cell phones to track people. One can't outrightly deny that covert methods of tracking and tapping cell phones do exist, but they are not available for general public. If you want to use technology to keep a track of your family members―children in particular, then cell phone tracking apps are your best bet. If you intend to do 'a lot more than that', we will advise you to hold your horses, as it is bound to spell trouble for you.