There's a particular approach to science called the scientific method.
With this approach to studying a system, we can form a hypothesis to explain an observation
or to make a prediction about how the system will operate.
Than we can test the hypothesis
using an experiment,
which is a highly controlled procedure
specifically designed to either confirm or refute the hypothesis.
If we make consistent observations that hold true across a wide variety of situations
we can form a scientific law,
which is a brief statement summarizing observations.
An example of a scientific law is the law of conservation of mass.
It states that in a chemical reaction, matter is neither created nor destroyed.
Since it pertains to all chemical reactions it is a very broad statement.
Notice that the law doesn't attempt to explain why we observe what we observe. 
It only focuses on what.
A scientific theory, on the other hand, attempts to explain why a system works the way it does.
To be considered scientific, theories and hypotheses must be falsifiable; that is, able to be proven false.
If it can not be tested and proven false, it cannot be labeled scientific.
For example, the statement "Life exists on an undiscovered planet"
cannot be labeled a scientific hypothesis, because it cannot be tested and proven false.
This is also why theories change with time
as our understanding of a system and the methods by which we can study that system advance.
In experiments the data we collect can be classified as qualifiable or quantifiable data.
Qualifiable data is subjective like color or shape.
Quantifiable data is measurable and needs to be accompanied with units.
If I asked you to tell me your height and you responded 193,
the number itself has no meaning without the units. It could be 193 feet or inches or centimeters.
Since there are so many units of measurement we also need a method to convert between units.
Later on we'll discuss dimensional analysis, which is the method we'll use to convert between units.
