The traditional way to prepare for tile is to lay a smooth mortar bed. It's a grueling procedure that requires skill, and many professional tilers still do it. However, it's much easier to install ceramic tile on a plywood or cement board underlayment instead. Like mortar, cement board is smooth and stable, and it's moisture proof. By itself, cement board is a suitable underlayment for tile.
Cement backer board is the best underlayment for tiles.
If you're installing tile over an existing concrete pad or on a traditional mortar bed, cement board would be redundant. However, you still need an underlayment to isolate the tiles from the concrete subfloor and prevent cracks. The most popular underlay for this purpose is known by its brand name: Schluter Ditra. It's a German product with indentations that accept the tile mastic, and the other side is a fibrous material that sticks to wet mortar.