Separation is the most familiar usage of subtraction and will likely be the easiest for your child to grasp. This involves the removal of one amount from another, as in, if I have three marbles in my hand, and give you one, I have two remaining.
Comparison involves taking two quantities and evaluating the difference in their amounts. If I have six pennies and you have four, I have two more pennies than you do. This again uses the basic arithmetical terms 6 - 4 = 2.
The Part-Whole concept refers to comparing a section of a total to a total, allowing a child to calculate a missing value. If there are ten toy cars in a playroom, and I have three of them, then there are seven cars somewhere else in the room.
Young children most effectively learn these concepts by interacting with physical objects. Acquisition of these concepts begin in the first few years of a child's life. Toys, drawings, as well as various sights outdoors all offer opportunities to illustrate these concepts to your child.