Yes. The voltage is determined by the chemistry inside the battery.
It means they come in different voltages to supply power to different things that require different voltages.
Batteries come with different voltages.
Yes you can. Though it is not recommended as it usually shortens the life of one of the batteries used.
The batteries for a micro RC helicopter come in different sizes and voltages. If you want to see a full product listing on those batteries, check out the website All-Battery.
Originally they were all special built devices, with many different voltages and current capacities. What we have today is just a few remnants.
You can connect batteries with different voltages in series (a 6 volt and a 12 volt would give you 18 volts), but never in parallel (difference of potential will create an arc, batteries could blow up).
It depends on the terminal voltage of the batteries, as different types of battery (or, more accurately, 'cell') have different terminal voltages. A car battery has Approx 12 V, so ten of them.
No you shouldn't do that. The voltages are different, and paralleling batteries aren't a good idea anyway. Why would you want to do this?
Batteries are connected in series to add their voltages together. For example, two car batteries in series will give twenty four volts.
The Makita Battery Charger is used for Makita batteries. This battery charger is capable of charging many batteries depending on the voltages for the batteries.
There are battery chargers designed to renew the batteries. For each tool the manufacturer usually provide the correct type with it or offer it as an accessory. By doing a little research you might find a multi- voltage charger which will charge batteries of different voltages from different tools, and perhaps from different manufacturers. If you have to replace the batteries, replace them with a type that the manufacturer of the cordless tool makes a charger for.
Batteries placed in series (which is what you're describing) add their voltages together. Thus two 1.5 volt batteries give 3 volts in series ... IF they point in the same direction. If they point in opposite directions, you get zero volts.