Short and sweet, An RO (reverse osmosis) is a type of utra-filtration that removes about 95% of the contaminents in water. The larger the ion, the better it removes it. Normally, salt rejection (sodium ion mainly), since it is smallest, is used to determine filtration rate.
Demineralizer or Ion/Exchange, as we use them is a nearly complete removal of contaminents from the water. When the resin nears exhaustion, sodium bleeds through so it is again used as a measure of water quality produced.
Number of gallons produced by an ion exchange resin depends on the quality of the feedwater, if it is dirty, then the ion exchange will exhaust quicker. With dirty water, putting an RO in front of the ion exchange bed will result in a longer life for the ion resin. That's mainly how they are used. If your feed water is relatively clean, an RO may not be as needed. I work at an industrial sized RO/DI (500gpm) and we use an RO because the feedwater is not so clean, other plants in the company do not use them due to cleaner feedwater.