Is the asthma medication "symbicort" (budesonide/formoterol) a preventative drug or a reliever?

It's called a...: ..."maintenance medication" - it does not prevent asthma as such, but it can prevent attacks from happening.

Preventative: Symbicort (budesonide and formoterol) is a maintenance / preventative inhaler.

Preventative drug: Medications such as symbicort (budesonide and formoterol) which contain and inhaled corticosteroid are used as maintenance (preventive) therapy to control persistent asthma. The formoterol component is a long-acting airway opener and may provide some initial relief of acute asthma symptoms; however symbicort (budesonide and formoterol) should never be used to relieve acute asthma symptoms (use your rescue albuterol under those conditions).

Controler Drug: Actually it is a controler combination inhaler as it contains inhaled steroids and a long acting bronchdilater and it is used twice a day to control asthma and albuterol oe xopenex (levalbuterol) is a resque drug for acute episodes.