Here's an analogy of electricity that oughta help...
Think of electricity like water... It's got a flow to it.
You have voltage. This is the speed at which the water is moving... Low voltage is a slow, lazy river... High voltage is fastly moving rapids.
Then you have current. This is the amount that is moving. You can think of this as the depth and width of the river.
Now let's go play in the water. You can stand in the slow, lazy river and it won't do anything to you... But if you stand in the rapids, you're likely to get blown over.
Principle 1... Too much voltage is dangerous. Luckily, USB chargers are all around 5 volts. So we're covered there.
Now let's do some actual work with this water... Let's get out a little toy boat and try and float it.
You can stick the boat in the ocean and it'll float.
You can stick it in the bathtub and it'll float.
But if you stick the boat in a puddle on the sidewalk, it won't float. You don't have enough volume of water to float there.
Principle 2: Too much current won't hurt things. Too little current and it won't do anything though. Things only take as much current as they need, any excess is just left over.
This is akin to running the phone strictly off the charger.
Now... Let's try and fill up the bathtub with water. I'll give you a teacup (Small current), a bucket (Medium current), and a large barrel (Large current) Which will let you fill the tub faster?
Principle 3: Batteries will charge faster with a larger current supply.
That's the basics of what you should know. Some will say that heat will damage a battery. However, the Droid tracks the temperature of the battery and it should slow th rate of current going to the battery if it detects too high of a temperature.
