EtherChannel is Cisco's solution to balance the load between multiple paths between two Ethernet based devices. As far as I can tell, it is only available on the Cisco Catalyst switch and Cisco routers with certain IOS images.
In a traditional switched network, when you connect two interfaces between two switches, you create a potential loop in which frames can travel forever. This is A BAD THING. With IEEE's Spanning Tree Protocol, this looping problem is avoided by blocking one of the ports to form a single, non-looping path. No regular traffic will be able to pass through the blocked ports. This also means that, even if you have two links of 200mbit/s each, you will still have an effective bandwidth of 200mbit/s between the two switches as one of the ports are down.
EtherChannel solves this problem by "bundling" up to eight ports between the two switches into a virtual interface. Traffic is load-balanced between these interfaces. This allows for the available bandwidth of all the links to be used while allowing for redundancy.
EtherChannel is usually deployed as an uplink to other switches when there are no higher speed interfaces available. It is low-cost if the equipment already exists.