The DHT11 is a low-cost temperature and humidity sensor. It isn’t the fastest sensor around but its cheap price makes it useful for experimenting or projects where you don’t require new readings multiple times a second. The device only requires three connections to the Pi. +3.3v, ground and one GPIO pin.
The device itself has four pins but one of these is not used. You can buy the 4-pin device on its own or as part of a 3-pin module.