What is a Field Sobriety Test?
In many states an officer will administer a field sobriety test (FST) if he has reason to believe you are under the influence of alcohol or drugs.
It is a physical and mental test used to determine if the driver can operate his vehicle safely.
There are many different versions of the FST. Here are some of most common tests:
The Walk and Turn – The suspect has to walk heel to toe in a straight line then turn around and walk back in a straight line.
The One Leg Stand – An officer will give you a time to balance on one leg.
The Romberg Test – This is a modified position of attention test. The officer may have you close your eyes, tilt your head up, put your feet together, etc.
A Memory Test - Such as reciting the alphabet.
The Horizontal Gaze Test – The police officer will have the suspect follow an object with only his eyes left to right. 
A field sobriety test is most effective in measuring how impaired an individual is. They are not used to determine BAC (blood alcohol concentration) levels.
Many FSTs use hand held breathalyzer devices to measure blood alcohol levels. However, these are not to be confused with the more advanced chemical tests at police stations called EBTs (evidentiary breath test) that employ infrared technology.
FSTs and the hand held breathalyzer devices are used to determine if there was probable cause for an arrest.
Some states allow them to be used as evidence others do not. If you refuse a breathalyzer or field sobriety test in some states they can use that against you in court as a sign of guilt.