John Palmer explains it best in his book on this page

"... the total amount of sugar is constant, but the concentration (i.e. gravity) changes depending on the volume. To understand this, let's look at the unit of points/pound/gallon. This is a unit of concentration, so the unit is always expressed in reference to 1 gallon ("per gallon"). In mashing, you are collecting "x" number of gallons of wort that has a gravity of "1.0yy" that was produced from "z" pounds of malt. To calculate your mash extraction in terms of ppg, you need to multiply the number of gallons of wort you collected by its gravity and divide that by the amount of malt that was used. This will give you the gravity (points per gallon) per pound of malt used."

and on this page

"For an average beer yeast, a rule of thumb is that the FG should be about 1/4 to 1/5 of the OG. For example, a typical beer OG of 1.040 should finish about 1.010 (or lower). A couple of points either way is not unusual."

Also this page for extracts http://www.howtobrew.com/section1/chapter3-4.html