The amount of voltage a device requires is typically dependant on it's battery. Different types of batteries put out different voltages and the devices powered by them are typically designed to operate on that voltage to prevent wastage.

In the case of most cell phones that use lithium Ion or Lithium polymer batteries, the voltage is typically 3.6V but can vary depending on the number of cells and how they're arranged (my notebook has a lithium ion battery but it runs at 14.4V)...this is no different than your car battery. Each cell generates 6V and within the battery there are several cells arranged to product 12V.

In the case of the universal chargers, they work in a couple of different ways. There are the types that are specific to certain types of phones and put out a voltage which the phone will accept. The perfect example of this are the Energizer chargers (the type I'm the most familiar with).

These devices are quite simple and use an integrated circuit known as a DC-DC Charge Pump converter or a Boost Converter. I could ramble all day about how these little buggers work but the bottom line is that low voltage goes in, higher voltage comes out, and heat is generated as byproduct and wastage. This is how a single 1.5V AA battery can charge a device which requires 5V (like an Ipod, or a Motorola or Blackberry phone). You can get these chips in varying input and output ratings but they're not super efficient, relegating them to temporary/emergency uses...or in applications where efficiency isn't important.

You can even make them yourself. There has been a DIY following behind these for a long time. Personally I'm lazy so I just modify the energizer ones to charge anything and everything I own which will charge off of 5V buy installing a female USB port and then buying or making USB charge cables for all my gadgets.

The following is a link to a very talented DIYer who created one of the first home brew chargers and sells kits to build your own in an Altoids Tin: http://www.ladyada.net/make/mintyboost/