CFL, standing for compact fluorescent lamp or light, is an energy saving fluorescent light designed as a replacement for older technologies such as halogen and incandescent lamps. These bulbs use between 70-80% less energy than incandescent and other traditional lights, and last between 6000 and 15,000 hours, easily beating the 1000 hours of an incandescent and 2000 hours of a halogen.
How do CFLs work?
Just like all fluorescent lamps, they produce light by sending an electric current through a tube containing argon and a small amount of mercury vapour. When the mercury vapours are excited, they produce ultraviolet (UV) light, which in turn causes the coating of the lamp to fluoresce or glow. Hence the name! 
CFLs contain marginally less mercury than the older fluorescent lamps, but still, need to be handled carefully if broken. 

Types of CFLs

There are two types of compact fluorescent lamps: screw-in and plug-in, also known as self-ballasted CFL and Non-integrated ballast CFL. As you may know, a ballast helps regulate the voltage, current, and more.
Non-Integrated Ballast technology, also known as a “plug-in” type, will require you to purchase or already own a ballast for it to properly operate.
In comparison, an Integrated Ballast type bulb, aka the screw-in CFL, is a direct retrofit replacement for halogen and incandescent lamp. This is what CFLs have been designed to do and do it well. Screw in CFLs can typically be directly placed into the existing screw socket.

Additionally, CFLs come in a variety of bulb styles, here are a few of the common types: 
Bulb and candle shaped CFLs are actually spiral CFLs under a shaped covering that allow the light to get through.   

Things to Keep in Mind

CFLs aren’t risk-free. While they are considerably better than incandescent and halogen bulbs, these bulbs still produce UV rays, although minimal, which may irritate the eyes. The mercury contained in them requires the user to properly recycle them at end of life, as they are damaging to the environment if not properly disposed of.   
Other issues include the time it takes these bulbs to light up – sometimes needing up to 3 minutes to reach their full lumen after being switched on. 

So, is it worth it?

Despite the drawbacks mentioned above, CFLs are still much more environmentally friendly than traditional incandescent and halogen globes, and with their longer life and lower, energy saving power consumption, CFLs are a definite winner against the older technologies.
That being said, if using a globe with mercury in it is a concern, consider looking into LED lighting!