Metric Volume
These are the two most common measurements:

A milliliter (that is "milli" and "liter" put together) is a very small amount of liquid.
Here is a milliliter of milk in a teaspoon.
It doesn't even fill the teaspoon!
If you collect about 20 drops of water, you will have 1 milliliter: 
20 drops of water makes about 1 milliliter 
And a teaspoon can hold about five milliliters: 
1 teaspoon of liquid is about 5 milliliters 
Milliliters are often written as ml (for short), so "100 ml" means "100 milliliters".
Here we have 150 ml of milk in a measuring cup.
It doesn't say "150" ... it says "50" ... but it is halfway between 100 and 200 so you can figure out it is 150 ml.
Liter
A liter is just a bunch of milliliters put all together. In fact, 1000 milliliters makes up 1 liter:
1 liter = 1,000 milliliters
This jug has exactly 1 liter of water in it.
Liters are often written as L (for short), so "3 L" means "3 Liters".
Milk, soda and other drinks are often sold in liters.
The next time you are at the store take a minute and look on the labels to see how many liters (or milliliters) are in each container!
Liter is abbreviated L (some people use lowercase l, but that looks too much like 1).