120v or 240v what's the difference?

A switchable ballast has 120v or 240v....How does one know which to use? Please explain. Thanks.

Is there a manual switch on it to set the voltage? or does it do it automatically?

120v or 240v will depend on where you live. I believe Canada uses the same setup as the US and should be 120v. I'm not absolutely sure though, so check some other appliances around your house and look for the label that states it's required power input.

a normal wall socket is 110/120v in the US. 220/240 volt is what your dryer plug uses (also your stove, and/or other appliances that require a large amount of amperage). It is also what is used in most of the rest of the world.

the advantage of running 220 volt is that you can run much more amperage on the line, which means many more lights. On a single 15-20 amp circuit in your house, you really should not be running more then 2 lights at the most. Depending on what else is using that breaker, it might pop once in awhile.

To use 220v in the US, you must use a box that splits the 220v connection into many smaller 220v connections that fit the 220v slant eyed cord. This is superior, but only necessary if running a lot of lights.