In New York they call them gypsys, in Philadelphia they call them jitneys, here in Baltimore we call them hacks. In other words, cheap, unlicensed taxi service. Everyone who has spent any time in the city has either used a hack or witnessed someone trying to catch one. Pay attention and on any given night you'll see people walking along the street holding their index finger while motioning downward until a car pulls up and scoops them away.
Hacks fill a needed gap in cab service in the inner-city. Most professional cab services prefer downtown trips or the longer, more lucrative trips to BWI. Hacks can be used to get to a relatives' house, run an errand or get to the local grocery store. But though they fulfill a real need everyone should still be aware of their risk they pose. They are illegal and therefore unregulated and because of this women in particular should take caution.
Additionally, there is no guarantee that a hacker even maintains auto insurance. In a city with where hit and run are common this poses particular risks. For instance if you get involved in a hit and run in hackers car and the hacker has no insurance you're probably out of luck.
Because of the risks hacks should probably only be used in the most dire of stations. If you do take them the following steps should be taken at a minimum: 1) take a photo their license plate; 2) pay attention to the hackers' car; and 3) pay attention to their face. In the days of Uber and Lyft hacking is probably a little less common but it probably isn't going anywhere anytime soon.