Systems of Measure

Systems of measurement have come a long way since the beginning of time.  People used to use hands, or feet, or arms length to measure things.  When you measured something you would say it is 5 hands long, or 6 hands long.  We have come a long way now with our English units for length that are inches, feet, yards, and miles.  When it comes to math, using the metric way is a lot easier than using the English way. 
Metric units are centimeters, meters, millimeters. 
Using centimeters is much easier because you are counting each little mark.  that stands for a point.  you can have 3.9 centimeters, whereas you would have to have 3 and 4/16ths.  It makes it much harder to have to convert them into decimals, and then add things up to get the right measurements for shapes.

When you look at the ruler you can see how much easier it is to count in centimeters. 
Measurement is used for many things, it is used to measure your body length as a baby, walls in a home, and everything else you do. 

Here are some measurement websites to help with understanding it.
http://www.aaamath.com/mea.html

Here are some good worksheets to help in your classroom with measurement.