Precision means how much detail is gathered by the measurement and how consistent the measurements are. A device that measures to .001 inches is more precise than one that measures only to .1.
Accuracy is how close to the RIGHT "answer" the measurement is. If something is really 10 meters long, and the measuring stick finds it to be 9 meters long, then the measuring stick is not accurate.
Thus, precision is an indication of detail and accuracy is an indication of correctness.
Naturally, it is best if a measuring device is both accurate and precise. However, something could be accurate but not precise or it could be precise but not accurate.
If you use a ruler to measure the length of a block, you will be able to pick a measure between (for instance) 23 millimeters and 24 millimeters. Using a different measuring device (for instance, a micrometer or vernier caliper) you could measure between much smaller increments. You would be able to read a measure between 23.4 millimeters and 23.5 millimeters. Thus, the second instrument would be more precise than the simple ruler.
A scale could be very precise and read the weight of the sample to be 24.54 grams. However, if the sample really weighed 26 grams, the scale would not be accurate.
Accuracy and precision are independent of each other. The ruler (or any measuring device) can be accurate, but not very precise. The scale can be precise but not accurate.
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