For other areas, like a floor or a kitchen counter, you should use an acrylic water-based sealer. Make sure the area is clean and dry before reapplying a sealer. These products clean primarily by acid and are eating your grout every time you use them. Instead, go to your local tile store and buy a neutral cleaner. There is a certain amount of maintenance with tile, but if you keep it clean it will look beautiful for many years.
Cement board is made from aggregated slurry with a layer of fiberglass mesh embedded into each side.
It acts like a single unit and is screwed or nailed down. If the walls or the floor it's being attached to are uneven or out of level or plumb, then the tile work will be out of line by the same amount. A mortar bed application is a solid unit of sand and cement mix that allows you to level out floors or plumb up walls so you can install the tile on a flat, true surface. A mortar bed is a far superior application. You need to use a masonry bit on your drill.