Beginners Guide: What is this all about?

Battlemechs are giant, stompy, weapon bristled robots, powered by a fusion reactor and controlled by men and women called “Mechwarriors”.

The entire story takes place in the 31st century, where mankind is at war once again. The interstellar space surrounding Earth (or the so called “Inner Sphere”) is divided into five major Houses and many independent systems. Those five Houses were constantly at war, until a new superior force appeared on the field: the technologically advanced Clans. The Clans had previously left the Inner Sphere in a mass exodus hundreds of years ago. They are now back to reclaim what is rightfully theirs: Terra.

If you want to know more about the awesome background of Battletech I suggest you take a look here.

To summarize it briefly: Battletech started as a tabletop game many years ago. After its success many novels and computer games were released to gain even more success. MechWarrior Online is the latest release in a long standing Mechwarrior history.