Ion drives are nearly ubiquitous. All spacefaring races end up inventing ion drives, and thanks to their high efficiency and abundant fuel source, they have ended up being one of the most common methods of propulsion in the galaxy.
The theory behind an ion drive is simple: an ionized fuel (typically hydrogen, or deuterium) is accellerated by a magnetic field, out some sort of nozzle. This, in turn, creates thrust in the opposite direction.
Many, many, many different variations on this basic concept exist. Most involve turning the ionized gas into a plasma and then using various techniques to accellerate that plasma to significant percentages of the speed of light to generate massive accelerations. Others ignight the gass, working more like a traditional rocket engine. They all have various applications and usages, but at the end of the day they are all forms of an ion drive.
Most modern ion drives are a variation on the plasma drive mentioned above. However, instead of just using magnetic forces to accellerate the plasma, most use some gravity manipulation to accellerate it even faster, greatly increasing the thrust without taking the same amount of power a purely gravitic drive would take to do the same work.
Since ion drives are so efficient, most ships have ion drives as their primary form of acceleration. This leads to a tendency to put several large ion engines on one end of a ship to help accellerate it towards it's intended destination. Early designs included ion engines on the "front" of the ship as well, but it was realized it was more efficient to simply turn the ship 180 degress and use the same "rear" ion engines. As gravity drives became more popular, ion drives remained the primary source of forward acceleration, often in tandem with a gravitic assist. The gravity drives, however, are most often used for movement in all other axis.