Science 122Laboratory 
Definition 
The least count is the value of the smallest scale division of the measurement instrument.

For each measurement made it is important to consider significant figures and to keep in mind the uncertainties involved in measurement. When scientists report the results of their measurements it is important that they also communicate how 'close' those measurements are likely to be. This helps others to duplicate the experiment, but also shows how much 'room for error' there was.
[Obviously, reporting a measurement of 7.956845712581 cm., made with a 25cent plastic ruler from the supermarket would be absurd. Similarly, using an atomic clock to count minutes would be overkill.]
Definition 
A significant figure (abbreviated s.f.) is one that has been measured with certainty or has been 'properly' estimated. The significant figures in a number includes all digits as read from the instrument plus one doubtful digit.

Examples:
The maximum error in a measurement is expressed as a plus or minus value of one least count.
For example, the length of a wood block is measured to be 15.4 cm +/ 0.1 cm. The maximum error given as + 0.1 cm means the length is between 15.3 cm and 15.5 cm. It also means that the measurement was made with a ruler with smallest marks of 1 centimeter.
You cannot increase the precision of measurement by doing arithmetic operations such as addition, subtraction, multiplication, division. This can be confusing because the calculator will return a large number of decimal places, especially when performing division. 
Example: 12.363 rounded to 4 significant figures is 12.36
Example: 15.47 rounded to 3 significant figures is 15.5
Example: 642 rounded to 2 significant figures is 640
 Example: 240 has 2 significant figures
 Example: 1000 has 1 significant figure
One exception is when a zero is obtained by rounding
 Example: 249.8 is rounded to 3 significant figures to become 250. Here the zero is significant.
Another exception is when the zero represents the estimated final digit. This is ambiguous because when you see the number as written it is not clear what that final zero represents.
LENGTH  

Unit  Abbreviation  Meters 
MILLIMETER 
mm 
0.001 
CENTIMETER 
cm 
0.01 
METER 
m 
1.0 
KILOMETER 
km 
1000.0 
MASS  

Unit  Abbreviation  grams 
MILLIGRAM 
mg 
0.001 
GRAM 
g 
1 
KILOGRAM 
kg 
1000 