I consider my friend and former roommate a problem drinker. At parties and every Friday/Saturday night he would get completely drunk. Sometimes after work he would crack open a beer or two after he had dinner.
However he would never get drunk during the week and if there were mitigating circumstances he wouldn't drink at parties. The guy had an on/off switch for sure.
Alcoholics have a mentality that we drink because we have to. I think problem drinkers have the mentality that they don't have to but they want to if they can.