What is retract speed & length? On a 3D XYZ printer extruder, I found two terms I have no idea about:   


retract speed
retract length


What are these and why/how should they be set?
Retraction is the reversal of the direction of the filament and is generally used when moving from one non-contiguous point of the print to another, in order to prevent stringing and oozing of the filament. If retraction is not employed then the filament still coming out of the nozzle, after the last point was printed (and paused), will stretch, thus creating a fine string, as the print head is moved to the next position where printing is to recommence.

The retraction speed is the speed at which the filament is retracted, or pulled back (by the extruder stepper), and the retraction length is the amount that is pulled back.
These settings are dealt with in the XYZware User manual, on page 43, section 11.5 Retraction

11.5 Retraction

11.5.1 Retract Length
In printing object, before large movement of print module, print
filament will be drawn back, such that slight negative  pressure
occurs  in  print  nozzle,  preventing  material  from  adhering  to
the  object  while  moving, improving surface quality of print object
11.5.2 Activate Threshold
Such  setting  will  allow  users  to  set  up  retraction  mechanism
activation  style.  For  setup  mode,  users  usually specify the
minimal print module movement distance for retraction mechanism
activation
11.5.3 Lifting height for extruder withdrawal
After retraction, the print module will be elevated slightly with such
setup value. Such action prevents material from adhering to the
object, and makes a more orderly final print stop point. However, it
should be noted that excessively large elevation will extend print
preparation time for the next print layer, and portions of angles may
results cooling and difficult to bond conditions between layers
11.5.4 Add Extra Filament after Travel &lt; Retraction
Material  compensation  may  be  used  to  improve  upon  holes  or
poor  extrusion  due  to  excessive  extruder withdrawal

Retraction speed isn't dealt with in the above section though. In section 3.3, of the XYZware Pro. User Manual, it is mentioned:

Retract Speed
The speed for pulling filament backwards. Refer to the function introduction in the next chapter for more about retraction.
Hint:
Cooperation of retraction speed and other print speeds will affect feeding stability directly in printing. A print speed slightly faster than the retraction speed would prevent material squeeze from interrupt.

However, the manual doesn't then go on to give any setting. However, the default settings should suffice, unless you are experiencing issues with stringing and/or oozing.
A further explanation can be found here, Stringing and oozing:

Reason 2: Retraction Length
The retraction function includes two setting options. One is
retraction length and the other is retraction speed. The retraction
length determines how much melted filament will be pulled out of the
nozzle. In general, the more plastic that is retracted from the
nozzle, the less likely the nozzle is to ooze while moving. As for the
issue, the default setting in the expert mode is enough for you to
solve the problem. If you encounter stringing with your print job, you
can increase the retraction length slightly to test again to see if
the performance improves.
Reason 3: Retraction Speed
The retraction speed determines how fast the filament is retracted
from the nozzle. If the speed is too low, it will make no difference
to your print job., the melted filament will still drop down through
the nozzle and leave on the model. On the contrary, if the speed is
too fast, the filament will be back to the nozzle and cannot be
extruded out in the next movement of printing. As for the retraction
speed setting, users can reserve the default setting which is perfect
for almost every models.

Testing your settings
As 0scar has reminded me, there are a good number of retraction test prints available (cubes, towers, bridges), which will help you check that your settings are adequate. These prints provide models that have a lot of print breaks (points between which printing is paused and then resumed), which can cause stringing to be exhibited. See RepRap Wiki - Oozebane:

Oozebane
Objective: stop material oozing out of the nozzle during
'non-printing' moves.
Many extruders will emit (ooze) plastic even when the extruder motor
is not turning. To overcome this your slicing software needs to
'retract' the print medium during head movement when not printing. The
retraction creates negative pressure within the hot end heating
chamber which effectively sucks the print medium back up through the
nozzle, stopping it from oozing.
Calibration Object: oozebane-test.stl
The calibration object prints two towers about 30 mm apart. The head
must move between each of the towers at each layer. If your printer is
not set correctly then you will see many fine filaments (or strings)
between the two towers. You can eliminate these filaments by
eliminating ooze.
Calibration Object 2 (Variable sized towers for testing ooze): variable_size_ooze_test_nobase.stl
This is a simple model to help tune reversal parameters for a stepper
extruder (using much less filament before actually testing the
ooziness). It consists of a number of towers with different
thicknesses, with different spacing between each tower. A well-tuned
bot should be able to produce even the smallest towers.

A simple google search, thingiverse retraction, shows up a lot of examples, such as:

10 Minute Mini Calibration Test for Oozing/Retraction at Different Distances
4 Cube Retraction Calibration
Retraction Tower Test or String retraction tower test


Check out the following (suspiciously similar) tags1, for even more examples:

retraction_test
retraction-test
retraction


Additional note for Bowden setups
As Trish notes in the comments:

According to my experience2 it is generally a good idea to add 2 to 4 mm of retraction to a bowden setup in comparison to a direct drive when dialing in the perfect retraction. This is, because some distance is &quot;eaten&quot; by the flex of the Bowden tube.


1 It seems as if Thingiverse could benefit from tag synonyms
2 and Thomas Sanladerer's advice during a stream