These two words do mean the same thing in general. However, "reduce usually refers to the action taken by an agent, while "decrease" usually refers to the drop in quantity itself. Therefore, "reduce" is usually used BEFORE the object and "decrease" is used AFTER the object:
Example:
"Tylenol will reduce fever." (used BEFORE "fever")
"If the fever does not decrease within two hours, call your doctor." (used AFTER fever)
"To reduce your risk of certain cancers, quit smoking." (used BEFORE "risk")
"If you quit smoking, your risk for cancer will decrease." (used AFTER "risk")
For this reason, you will see "reduce" preferred over "decrease" in most situations and phrases when the DOER is the focus (i.e., when placed before the object):
REDUCE:
"to reduce debt"
"to reduce calories / caloric intake"
"to reduce weight" (diet)
"to reduce use" (of some natural resource or medication)
"to reduce inflammation / fever / etc." (medical conditions)
"to reduce risk / chances"
"to reduce heat" (when cooking)
"to reduce spending"
"to reduce taxes / interest rates / etc."
There are others.
Also note:
The noun form of "to reduce" is "reduction".
Ex.: "a reduction in daily calorie intake"
The noun form of "to decrease" remains "decrease".
Ex.: "a pay decrease"
Be careful with numbers and mathematics, where "reduce" and "decrease" do NOT mean the same thing. Here, " to decrease" means to subtract or make smaller, while "to reduce" means to state a number (usually a fraction) in the simplest way.
Ex. Given the fraction 2/4
1. To decrease this might be to have only 1/3 or 1/4.
2. To reduce this would be to express it as 1/2 (the same amount as 2/4).