Good question - most English speakers would have a problem explaining the difference between the two words. I did a Google search of "use vs usage", and found the following explanation at: http://coachdes.wordpress.com/2005/10/24/english-use-and-usage/
USE vs USAGE
The noun ‘use’ comes from the verb ‘use’, meaning to employ for a given purpose or put into action, and larger dictionaries will list many variations and adaptations of that basic meaning. Examples are: ‘I use a keyboard to type in these words’ ‘I use a knife and fork to eat my dinner’, ‘I use short words in speaking with small children, because they probably won’t understand long words’. So the noun ‘use’ (with the ‘s’ as in ‘goose’, not, as for the verb, as in ‘cruise’) means a given purpose or application. Examples would be: ‘The English language is in common use around the world’ , ‘I put my keyboard to good use’.
For the noun ‘usage’ the basic dictionary definition can look pretty much the same as that for ‘use’, but with ‘usage’ there is a sense of ‘continued’ or ‘common’ use. And with language, the distinction is that ‘usage’ is the way the language is actually used, as distinct from what might look correct if you try to construct a sentence or phrase from a dictionary and grammar book. Examples would be: ‘Although old-fashioned grammarians say you should never split an infinitive, that is done every day in common usage.’ and ‘I was taught at school that every sentence must have a verb, but actual usage shows that many excellent writers include in their work ’sentences’ without verbs, such as ’His arrival at any gathering was always a dramatic event. Bold. Arresting.’