Condenser microphones are the delicate cousins of dynamic microphones. These can break easily with aggressive use and are not meant to be handled by your hands as you speak into it, so don't grab them and wild-out like it's a dynamic or thats your however-many-hundred-dollar mistake. In order too operate, condenser microphones need a source of power, namely phantom power. Unlike dynamic microphones, they will almost always have XLR connections. Other features a condenser microphone may include are built in phantom power by way of battery, a bass filter cut-off switch or high-shelf cut-off switch to automatically cut the lower unneeded frequencies from a recording. Another feature is a red LCD light indicating that the microphone is connected to an operating phantom power source, and also perhaps a switch to turn the microphone on and off.
The main difference between dynamic and condenser microphones is the quality. Whereas a dynamic microphone will record what's close to it, a standard quality condenser turned up will record the whole room plus some. Aggressive rappers do not need to spit flows loudly into a condenser as it will pick up even the faintest of sounds. Treatment for an overly sensitive condenser is the pack the room in professional quality acoustic foam, if not done you may pick up sounds from automobile traffic, children screaming and playing (seems like they don't know how to whisper), dogs barking, neighbors arguing as if it's right in front of you, and that's just naming a few of the types of distractions and audio anomalies.
They are many different types of condenser microphones, what you will be wanting to record your flows on is a large diaphram condenser microphone (the recording style of cardiod, omni-directional is your choice) as the other main alternative is a "shot-gun "condenser microphone which is usually in the style of a 1-inch width-wide, tube cylinder shape with the part to record a flattened wire screen.
Because they are easily damaged if handled improperly, condenser microphone usually sit in the comfort of shock mounts with a pop screen filter nearby. A condenser microphone can range from $ 150 to $ 2000 + depending on the brand and quality, of course the more expensive the microphone generally the better. Good brands to look for are Rode, AKG, Audio Technica.