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Police will ask a person to perform "field sobriety tests" to prove "probable cause" to make an arrest for a DWI. These tests are done to show you didn't have your normal use of  mental or physical faculties.

The (National Highway Traffic Safety Administration) NHTSA provides manuals for police officers to properly perform "field sobriety tests". If the tests aren't performed according to the manual, they are unreliable. The HGN, Walk and Turn, and One Leg Stand tests are the "standardized" tests in the manual.  Police sometimes use other tests that are not "standardized" by anyone.

1) HGN:  Horizontal Gaze Nystagmus test.  This is where the officer holds an object (usually a pen-like object), and asks you to hold your head still and follow the pen with only your eyes. They are looking for:

1) lack of smooth pursuit,
2) nystagmus (jerkiness of the eye), and
3) nystagmus before 45 degrees. 

The test must be performed correctly to be admissible.

2) Walk and Turn (WAT): The officer has you walk an invisible line stop, turn, and walk back.  You are given a series of instructions, told to stand in an unnatural position during the instruction phase, and then told to begin.  The officer grades you based on your ability to follow directions (mental), and your ability to perform the task (physical). 

The officer doesn't tell you what he will be looking for prior to the test.

3) One Leg Stand (OLS): This test is similar to the Walk and Turn, in that you are given instructions, asked to stand in an unnatural position during the instruction phase, and told what to do.   You are to stand on one foot with the other foot lifted in front of you.  Usually the officer continually tells the subject to lift their foot "higher, higher" for 30 seconds.  It often causes someone to lose balance when they continuously have to raise their foot higher and higher.  Again, the officer will test the ability to follow directions (mental), and the ability to perform the test.