What is the difference between charging batteries in series and parallel?
Will it take a bit more than 3 hours to charge a 900 mAh AA battery?
  


Yes, due to chemistry losses given out as heat.


  
  What if I have 4 AA batteries in series? Will it still take 3 hours?
  


Yes, provided the charger has enough "headroom" to drive 300 mA through the batteries as their voltage will rise during charge.


  
  How does voltage play into all of this?
  


The charger must be capable of a higher voltage than the batteries if current is to flow from the charger into the batteries. For example, your car alternator gives out about 14 V to charge the 12 V car battery. (Be careful though: your car is using a lead-acid battery which is charged at constant voltage whereas your AA cells require constant current but still with higher voltage.)


  
  I am planning to use a solar panel to charge 4 AA 2300 mAh batteries. The solar panel is 6 volts and 100 milliamps.
  


As discussed 6 V might not be enough. Even if it was, at 100 mA charger current and, say, eight hours good sunshine per day it will take at least three days to charge your cells. If you have to split them into two 3 V pairs on account of the voltage you might only get 50 mA per circuit. Charge time will be six days.