Cell phones are great inventions, they allow you to communicate with someone, almost anywhere you are. I say almost because we all know that cell phones don't work everywhere, in fact cell phone companies talk about more bars in their ads. They want you to use their service because it's available in more places. Sometimes cell phones won't work in certain buildings either. A cell phone antenna is what you need in both situations.
Cell phones have antennas built into them, but sometimes they're not enough to get the job done. You can buy an external antenna that plugs into the data port of most phones. These antennas are most suited to car installations, most of the times you'll get cell service dropouts in remote areas. Not every phone has the ability to use an external antenna, be sure to see what phone your antenna works with when purchasing. Don't bother buying the stick on cell antennas, they don't work and in some cases make your cell reception worse. They don't connect to any of the phone's internal circuitry so there's really no way they can work.
Another type of cell phone antenna is the repeater or cell signal booster. These can work in cars but they're most suited to buildings that get poor reception. These antennas plug into the wall (or cigarette lighter in the case of a car) and take the weak signals they receive and boost them. If you're in a building with poor reception then you can put the base station inside an an the actual antenna outside to capture the cell signals, then boost them inside so that the building will get normal cell reception.
So if you use your cell as your primary phone, or you travel a lot and can't get bars on that new pink cell phone you just got, then maybe a cell phone antenna is what you need.