The DHT11 is a basic, low-cost, digital temperature and humidity sensor. Inside it has a capacitive humidity sensor and a thermistor to measure the surrounding air, and spits out a digital signal on the data pin (no analog input pins needed). Its fairly simple to use, but requires careful timing to grab data. Luckily Adafruit has put together an excellent library (listed under this products Resources)!

The sensor can be polled every two seconds and will read between -20 and 60°C with a ±2°C accuracy. Humidity readings range between 5 and 95% with 5% accuracy. For a wider range of temperature and more accurate humidity monitoring check out the DHT22 or the AM2302 sensors.