You probably learned about significant figures (or "sig figs") in high school chemistry because that is where you really began to quantify your experiments. If you haven't learned sig figs before don't worry, they are just a fancy name for something that you already know. If you have seen the topic before then this short refresher will not bother you at all.
Precision
Significant figures are all about precision. When a scientist, or anyone, states a value or quantity, the way that they state it tells you something about how precisely they know what they are talking about. For instance, here are two clocks that state the time at which you arrived at this page of the lab.
Accuracy
Accuracy is a measure of how certain you are about something. A high degree of precision does not ensure accuracy since more significant figures does not change the fact that a measurement may be flawed. The measure of a mile at 5280 feet is more accurate than if it were measured as 5270.0598 feet. 

If you asked someone the time and they told you, " ,". Which clock must they have read? Clearly, they must be reading the digital clock because it has precision down to seconds.

They could have been less precise and said, " ," but the fact that they stated more figures that had significance communicates to you that they must have something more precise than a face clock.

This concept will affect your work as you learn scientific perspective this semester. 

Counting Significant Figures

A significant figure is any digit in a number when it is written in scientific notation.  So, the number 20452.89 has seven significant figures.  But the number 0.000098276 only has five sig figs, because when written in scientific notation it is written 9.8276e-5 and you lose all of the leading zeros.  Any zeros explicitly written after a decimal point are also significant figures. So, the number 2.850 has four sig figs while the number 2.85 only has three sig figs, even though the two numbers are mathematically equal.

Significant Figures in Calculations

The place where significant figures usually becomes important in this class is when you are asked to do a calculation. Your final answer should not be more precise than your least precise measurement. In other words, your answer should only have as many significant figures as the measurement with the fewest significant figures. Let's look at a simple problem.

Problem

Homer is 6.1 ft tall. Maggie is 1.7 ft tall. How many times larger than Maggie is Homer?

Solution

We must divide 6.1 ft by 1.7 ft. When I punch that into my calculator I get 3.588235294118.

So I should just write down and circle what my calculator says is the answer, right? No, because there are a lot of sig figs on the calculator screen that misrepresent our precision. We only know the two heights to a precision of two significant figures. Therefore, we can only know how many times taller Homer is to the same degree of precision. The correct answer would be 3.6 because it correctly expresses our precision. It has as many sig figs (two) as the 6.1 ft and 1.7 ft that we were given originally.

But, by writing down exactly what is on your calculator screen you would be telling me that you could measure their heights to a trillionth of a foot and I would mark you wrong.


The Moral of the Story

As you are going through this course think about the numbers that we tell you as well as the numbers that you hand in as answers. Scientists must pay attention to the accuracy as well as the precision of the ideas that they represent.