Charging devices - Voltage and Amperage
Note: Main OK point from below is that a power supply with the correct voltage rating and a higher than specified current rating will be OK to use in the vast majority of cases. The situations where this does not work are so rare as to be worth risking unless major safety or $ value concerns exist. Even then you can probably check with over current supply at correct voltage and see if problems occur. If the equipment is worth over 2 weeks income get the correct supply :-). 

Power = Volts x Amps = V x I  

Too high Voltage will do damage.
 Equipment may be damaged
 Equipment MAY shut down 
Slightly too high may be OK. YMMV.

Too low voltage may do damage.
 Slightly too low may work AOK or at reduced current.
 Too low may damage equipment but not usually.
 Too low may damage charger but not usually.

Correct voltage rating and too high current rating is USUALLY OK - equipment will take what it needs.
 A very few items of equipment will object in some way.

Correct voltage rating and too low current rating may cause damage.
 Slightly too low may be OK with computers an similar.
 Older equipment using power supply as charger will quite often work with current rating far too low and just charge slower.
 Too low current rating - equipment MAY load power supply down - MAY damage supply - MAY work - MAY just refuse to work. eg Most notebooks / laptops / tablets - typically with 15 to 20 Volt supplies - will not even try to charge if power supply voltage is more than a Volt or two too low