RFID stands for Radio Frequency Identification, and it does exactly that: provides identifying information on a tag that may both receive and transmit information at radio frequencies. This information can be anything, from your heartbeat to the name of your dog to the owner of a cow, and is useful for a huge variety of purposes. The strength of the signal from the tag to the receiver may also be used as a locator, especially when used with multiple lo caters to triangulate the position.
There are three types of RFID tags: active tags, which contain a battery and are constantly transmitting some sort of data such as vital signs, passive tags, which require external source such as a scanner to create a signal in an otherwise batteryless device, and then battery assisted passive, which function as something of a hybrid of the two in that an external source is required to activate the battery functions. Each of these have their nuances of use.
As you might imagine, this technology is best suited for smaller spaces, where the infrastructure is already in place to use it. RFID requires specialized scanners to read and transmit data, and without one specific to the proprietary receivers, there's no point. The dedicated infrastructure may be of great cost on a large scale, but on a small, localized scale, may be incredibly powerful for both tracking and for providing information.
That being said, they work brilliantly for hundreds of purposes in your every day lives, from automatically scanning highway toll fees to using Zipcars to use of public transportation to preventing shoplifting to IDing livestock to even identifying humans by passport—or implant. RFIDs serve for an incredible variety of purposes, and the number is just likely to grow.
Now, GPS is a very different beast from RFID. While it also uses radio waves to transmit data, it does so using, well, the global positioning system of 24 satellites, as opposed to specialized scanners here on the ground. Radio waves sent out from this system of satellites transmit their time and orbital data to receivers down on Earth. Using the data from multiple satellites, receivers can then triangulate their position relative to the satellites, and thus on the Earth’s surface.
GPS, thus, is best suited for tracking anywhere in the world—but because of the sheer distance of the satellites, the signal is weaker, and is easier to jam, or even just not get a signal. Civilian models particularly are not as accurate in certain situations as one might like, for instance at the bottom of a canyon or indoors.
Emergency homing beacons, car trackers or navigational devices tend to be the most well known civilian uses, which don’t require accuracy within a few inches, but happen on a large scale where no other infrastructure such as RFID or radio towers are set up. GPS is, by definition, global, and so the sort of tracking it’s best at happens on the scales of tens or hundreds of miles.