The Yo-Yo test series evaluate an individual's ability to repeatedly perform intervals over a prolonged period of time in intermittent exercise related sports. The endurance version of this test is designed to test The test evaluates a players aerobic endurance fitness.
There are 2 high level forms of the Yo-Yo Test:

• Yo-Yo Intermittent Test
• Yo-Yo Endurance Test (includes a 5-10 sec recovery break between bout).

There are also two levels of the test (Level 1 & 2), for each type each having a different starting speed and increments.

Comparison of scores for the above testing proceedures is not easy due to the wide range of variations in the testing format. However, below are some examples you could use.

• In a study of 13-14 year old spanish footballers, yo-yo test results ranged from 13.1 for centre fullbacks to 14.46 for wide midfielders (Assessment of anthropometric and physiological aspects of 13-14 years old Spanish footballers, by Juan Bautista Castilla Arroyo)
• Male top-class soccer players, playing matches at the highest international level, had a higher performance level on the Yo-Yo IR1 test (2420 m) than elite (2190 m) and sub-elite players (2030 m) as well as moderate trained soccer players (1810 m). (retrieved from soccerfitness.com July 07)
• A study of female Danish Premier League soccer players found they had a Yo-Yo intermittent endurance level 2 (Yo-Yo IE2) performance of 1,213 m +/- 90 m (n=23). (from Krustrup, P, Zebis, M, Jensen, JM, and Mohr, M. Game-induced fatigue patterns in elite female soccer. J Strength Cond Res 24(2): 437-441, 201).
• Soccer legend Kristine Lilly ran 49 shuttles for the YYIRT (10 second recovery periods) in September 2010 at the age of 39, setting the record for the US Women's National Team.

Yo-Yo Intermittent Tests
This test was developed by the Danish soccer physiologist Jens Bangsbo. Essentially repeated 40m (2x20m) runs with a recover period in-between. The Yo-Yo Intermittent Tests are similar to the commonly known Beep Test. However, in the intermittent tests the participants have a short active break (5 and 10 seconds for the intermittent endurance and intermittent recovery test, respectively). There are two versions of each Yo-Yo Intermittent Test, Level 1 (for beginners) and Level 2 (advanced players).  Below are downloads to the Nike version of this test.

Endurance Version (Of Intermittent Test)
Mark out three separate lines (using cones/flags) as per the diagram above; 20 meters and 2.5 meters recovery loop. The subject starts on or behind the middle line, and begins running 20 m when instructed by the audio cd. This subject turns and returns to the starting point when signaled by the recorded beep. There is a active recovery following each shuttle (out and back) period of 5 seconds takes place as the players walks/jogs around the end cone back to the starting position (middle line). If a player fails to make it back to the start line in the allocated time set by the audio CD the player receives a warning message. The player fails the test if they cannot make the subsequent signal (beep). A score is recorded and is a total distance of ground covered during the test before failure.

Recovery Version
This version includes a 5 meter recover walk/jog loop.

Yo-Yo Endurance Test
This test was also developed by Danish soccer physiologist Jens Bangsbo. Again there are two versions, Level 1 & 2 (a beginners and advanced level). The level one test is essentially the same test as the commonly known ‘Beep test’.
Following the directions from an audio DC the players run back and forth between two lines (indicated by cones) attempting to run the distance prior to a signal (beep) from the CD. The pace increments after a period. Players receive a warning if they do not complete a bout in the assigned time. The Level 2 test starts at a higher running speed and has different increments in speed. The athlete's score is the total distance covered before they were unable to keep up with the recording. The Yo-Yo intermittent test usually takes between 6-20 minutes for level 1 and between 2-10 minutes for level 2.

References:

Fitness Training in Football, a scientific approach - by Jens Bangsbo, publisher August Krogh Institute - Copenhagen University (December 1994).

Krustrup, P., Mohr, M., Amstrup, T., Rysgaard, T., Johansen, J., Steensberg, A., Redersen, P, K., Bangsbo, J. (2003)

Krustrup P, Mohr M, Nybo L, Jensen JM, Nielsen JJ, Bangsbo J. (2006) The Yo-Yo IR2 test: physiological response, reliability, and application to elite soccer. Medicine & Science in Sports & Exercise. Sep;38(9)