XBee Module - ZB Series S2C - 2mW with Wire Antenna

Series 2 modules cannot talk to Series 1 modules so if you already have some S1 type XBees you may want to stick with them. The S2 modules are not necessarily 'better' than S1 for many projects. They're just different as they use the "ZigBee" wireless stack instead of the 802.15.4. This makes them better for low power usage and advanced users who want a mesh topology (many XBees in a spread-out configuration) - but they are more difficult to use for basic point-to-point setups.
 

The XBee Module - ZB Series S2C - 2mW with Wire Antenna is a small radio communication module based on the XBee Series 2C (ZB) wireless platform from Digi, designed to make it easy to add short-range wireless networking (ZigBee) to embedded electronics projects.

It’s a wireless RF module operating in the 2.4 GHz ISM band that implements the ZigBee mesh networking protocol (IEEE 802.15.4). This lets it send and receive data wirelessly between microcontrollers, computers, gateways, or other XBee devices without wires, forming point-to-point, point-to-multipoint, or full mesh networks.