Radio units for transfering Serial data. Moderately easy interface with microcontrollers (aka best place to get started if new to wireless). cost:$25-50.
Xbee and xbee pro are wireless radios usually used in large quantities on the same channel for take spread data and surveys. Use TX & RX lines on the arduino to communicate.
Pin layout willl not fit a breadboard, you will need either an adapter ($6) or solder onto the IC's top pins directly(it works just use leaded solder, but looks mad janky and unreliable :( )
XCTU to format channels and establish Master Slave (more tba). Usually one channel for all signals will do.

Pin Layout;
Left Side
top pin: Vcc
2nd pin: TX
3rd pin: RX
bottom pin: GND

For simple communication, that's all you need to know.

(tba more coming)

Xbee's range: 2500 feet.

Xbee Pro's range: 1 mile (with straigth line of sight outside.

Applications: RC cars, Quadrotors, long-ranged communications.