Precision and Accuracy
Precision refers to the closeness (agreement) of measured values, based upon some accepted standard of "closeness" (generally called a percentage error.)
Examples: 10.2, 10.6, 10.4
Allowing a 15% error (which is, by the way, rather large) these values are precise.
Accuracy refers to the closeness to an accepted value.
Examples: Measured values are 9.6 and 9.7. The accepted value is 9.89. Accuracy is good in this case, as is the precision. They are very close (within an average 0.24 of the accepted value) and closely agree with each other (only 0.1 difference.)
Example: 6.2, 6.4, 6.38 Accepted value: 10.27
Good precision, poor accuracy
Example: 6.3, 9.8, 28.12 Accepted value: 0.023
Horrible accuracy and the precision is even worse!