There are three general types of drives used in propulsion of interstellar space craft, the arcjet drive, ion drives, and pulse drives.
The arcjet drive is the most commonly used in the commercial and consumer industries and is a plasma-based propulsion system which finds its roots in the 21st Century VASIMR (Variable Specific Impulse Magnetoplasma Rocket) project. Banks of MHDGs (magnetohydrodynamic generators) are used to ionize and heat propellant fuel which is used to provide thrust. Arcjet drives are one of the oldest drive systems available and is still widely used because it is capable of using a variety of fuel sources. The higher the quality of fuel the greater the thrust provided, but in an emergency very simple fuels can be used. Fuel in general is plentiful and cheap and the drive engines are easier to maintain than the Ion or Pulse drives used by most military vessels. Its low thrust potential however makes the arcjet drive impractical for military use.
More common among military vessels is the Fusion Pulse drive or Pulse drive for short. The Pulse drive represents large scale application of fusion cell technology. Thrust is produced through igniting fusion fuel into plasma through a series of magnetic containers. Most Pulse drives are powered processed deuterium/tritium or deuterium/helium-3, which has become a sought after resource. There are multiple methods used by various drive manufacturers for this type of drive primarily focused on how the fusion reaction is contained and focused.
The most powerful drive used among the military and private interests is the Ion Drive. Officially these drives are called Axial Magnetic Ion Drives, but over the centuries have become known simply as Ion drives. Based on early 22nd century fission-fragment technology, the AMI drive utilizes compact thorium matrix ‘cores’ as fuel. These matrixes are actually composed of millions of threads of nanoparticles which are ionized as fission occurs. AMI drives consist of three or more ‘cores’ which are cycled through the reactor. These drives have the advantage of serving as both a power producing and thrust producing mechanism, as the ionized particles can be used to generate power or projected for thrust which reduces the need for MHDGs to be installed. While Ion drives produce higher levels of thrust than arcjet drives, the cost of operating them is significantly higher.