In order to get certain solutes across the phospholipid bilayer, transporters are required. This can be by way of either passive transport or active transport. Many transporters will move a single type of solute across the membrane, and such transporters are called uniporters. However there are other types of transporter that will transfer two different types of solute at a time, and antiporters are from this family of coupled transporters. In this family of transporters the transfer of one type of solute depends completely on the transfer of the other. When both are travelling in the same direction, it is referred to as a symporter, and an example of this would be a Na+/glucose transporter. Antiporters, however, work by carrying a solute into or out of the cell, using the electrochemical gradient of the second solute, which is travelling in the opposite direction. In other words, solute A will travel down a concentration gradient into the cell, releasing energy which is harvested by solute B and used as a driving force to be pumped out of the cell in the opposite direction, but within the same transporter. An example of such transporters is the Na+/Ca+ antiporter, driven by the electrochemical gradient of Na+ across the membrane.
The Na+/K+ pump, which typically pumps out 3 Na+ ions for every 2 K+ ions pumped in, is commonly thought to be an antiporter, however this is not necessarily true as both ions are in fact moving against their concentration gradient. 
- ↑ Alberts B, Johnson A, et al (2008), Molecular Biology of the Cell, 5th Edition, New York: Garland Science. Chapter 11, Membrane Transport of Small Molecules and the Electrical Properties of Membranes, pages 656-660