Accuracy and Precision:
Accuracy refers to the closeness of a measured value to a standard or known value. For example, if in lab you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate. In this case, your measurement is not close to the known value.
Precision refers to the closeness of two or more measurements to each other. Using the example above, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy. You can be very precise but inaccurate, as described above. You can also be accurate but imprecise.
For example, if on average, your measurements for a given substance are close to the known value, but the measurements are far from each other, then you have accuracy without precision.
A good analogy for understanding accuracy and precision is to imagine a basketball
player shooting baskets. If the player shoots with accuracy, his aim will always
take the ball close to or into the basket. If the player shoots with precision,
his aim will always take the ball to the same location which may or may not
be close to the basket. A good player will be both accurate and precise by shooting
the ball the same way each time and each time making it in the basket.