in layman's terms, what grumpy is saying, your displacement is the volume of the space between the piston at full up stroke and full down stroke.
let's say you have a 2.8 liter L6... Stock bore is 87mm (diameter of the cylinder) and stock stroke is 79mm.
Using grumpy's formula, BORE x BORE x stroke X NUMBER OF CYLINDERS x .7854, and plugging in the numbers, the result is 2,817,784 mm^3, or 2.8 liters.
So to stroke it to a 3 liter (hence the term, 3 liter stroker motor) the common thing to do is to replace the crankshaft with more aggressive lobes. This makes the piston rod travel farther up and down, and therefore the stroke (or the length piston has travelled from lowest point to top dead center) is increased from 79mm to 83 using a maxima diesel crankshaft.
Now, if you plug in the numbers using the same formula, you get 2,960,457 mm^3, or 2.96 liter stroked motor...roughly 3 liters.
When a motor is bored, it means the cylinder diameter is increased, and you would fit larger pistons into the block in order to increase the displacement without changing the stroke. Or you could combine both for optimal displacement. However, boring a block can only be done so much because of the thickness of cylinder sleeve walls. Too thin and you run the risk of cracks and breaking.
Hope that helped