Space travel, however, is rather complicated, depending on the sort of story you intend to write. If you write hard SF you may find yourself in a bind. Einstein--that unrelentingly intelligent bastard--basically makes faster-than-light travel impossible (theory of relativity and all). Yet there are so many different types of fictional, and real, methods of traveling in space. What methods are realistic and what methods are fantasy?
- Faster-than-light Standard & Lightspeed (i.e. at the speed of light)
Your typical, commonly used, rarely explained method. Basically, it literally goes faster than light by its own power and within real space--not a wormhole, hyperspace, etc.. This is the type of drive you can't really explain because it's so unbelievable anyway it would be a waste of your time, and the time of your readers. You'd find yourself in a conundrum if you did. Einstein made this obsolete as a realistic approach--though it sort of was fantasy to begin with--by postulating that the closer you get to the speed of light, the more fuel you need to close the gap. The problem is that you eventually hit a point where no matter how much fuel you add, the fuel's weight counteracts the push it would provide. You'd hit a point where you couldn't ever put enough fuel in the ship to go any faster. It was a lot more complicated than that, because it's Einstein, but you get the idea.
Generally speaking, your standard form of FTL drive has a fuel source that is tangible and accessible and in some way or another works like rocket fuel--meaning it burns away or dissipates and doesn't have any affect on the fabric of space.
I'm referring to your typical instant-to-light-speed drives. Strangely, these are also drives that are never really explained, mostly because you don't have to. Your audience simply has to assume that you can go from zero to light speed in less than sixty seconds and there isn't anything out of the ordinary about that. Maybe you'd come up with a really inventive way to make this work, but most likely you'll just resort to calling it an FTL drive or give it a fancy name and leave it at that. If the rest of your story is good it won't really matter anyway.
- Jump Drive
This may have a different name. You could call it a teleport drive. The idea is that your ship simply disappears and reappears somewhere else. These are also called "discontinuous" because the traveler never traverses the actual real-space distance. You simply poof into existence somewhere else--like a genie. These could be in many different forms--jump to anywhere, jump to a 'node' in a specific location, jump only certain distances, jump in known space, etc. Either way, it's all fantasy because, as far as we know, you can't just disappear and appear somewhere else.
- Fold Drive
This is somewhat like a jump drive, as from the perspective of someone watching it would look like the ship simply disappears and reappears somewhere else. Fold drives revolve around the idea of being able to actually fold space so that two points are put side by side. Think of it this way: take a piece of paper and poke a hole on each end (longways), then fold that paper until those holes meet up and put your finger through it. That's folding space. The concept is, well, probable, but not possible in our near future. The energy required would be considerable. However, since space is filled with events where things get screwed up from black holes and the like, it wouldn't be all that surprising to realize that folding space isn't all that hard...at least to the universe.
- "Trail" Drive
Think of this as being like a railroad. A train has to have tracks to get from one point to another. The idea behind a "trail" drive, or railroad drive, is that the ship could achieve FTL in one of two ways: along a fix structure that allows the ship to do so, or along a trail of materials that the ship could use to achieve FTL.
In the first case you have two problems. The first is figuring out how to use such a structure to allow a ship to break the light barrier, which, according to Einstein, is impossible. Perhaps if the structure manipulates real-space it my be possible. The second is that you would have to build this structure in the first place, which would take thousands upon thousands of years most likely. You need a lot of material and you need people to watch over the structure when it breaks.
The second case is probably even more ridiculous. If you were to put a trail of matter that could be used to propel a ship to the speed of light you would have presented several problems while solving another. While you have figured out how to get a ship up to speed without the ship having to carry its fuel source with it, you also have created a big problem: you can't do these travels all the time. You'd have to put the material there, which would take thousands of years, and you'd have to keep putting it there every time it is used. This would be a logistics nightmare. It would still be possible, but it is very unlikely that someone would waste the time to do this.
- Ion Drive
These are actually real today. We use them in satellites because they don't require much in the way of fuel. Currently our ion drives accelerate very slowly, however they can accelerate for a significantly longer period of time than combustible fuel sources, meaning that in the long run an ion drive can go faster. Here's a basic understanding of how it all works from this site.
Ion propulsion is a technology that involves ionizing a gas to propel a craft. Instead of a spacecraft being propelled with standard chemicals, the gas xenon (which is like neon or helium, but heavier) is given an electrical charge, or ionized. It is then electrically accelerated to a speed of about 30 km/second. When xenon ions are emitted at such high speed as exhaust from a spacecraft, they push the spacecraft in the opposite direction.
- Fuel Drives
I'm using a basic concept here. Basically any sort of drive that uses a combustible fuel--jet fuel, etc.--or any sort of fuel that we know of today that can be used to propel spacecraft. These sorts of drives would be impossible to use for long distance travel--i.e. out of our solar system--are quick, but can only be used in short bursts, and are heavy and dangerous. I don't think much more explanation is needed. We all should know how fuel works since most of us drive cars.
- Nuclear Drives
There are two types of nuclear drives--reactor and impact.
Reactor drives should be pretty easy to understand. Take a tin can, put a nuclear reactor in it, and you now have a tin can with a radiation packed rear end. You'd of course need radiation shielding for the crew and some sort of guarantee that the darn thing won't explode on you in the middle of space, since you can't exactly run away. In theory, though, it would be a long lasting propulsion device that could get you going relatively fast. Again, though, you couldn't do much in the way of long distance travel because it could take you decades to get to another star.
Impact drives are, well, interesting and crazy at the same time. The idea is that you have the ship in front and some sort of metal plate or a thick rear area. Then, you cause small nuclear explosions in the rear and the impacts shove the ship in whatever direction you're wanting to go. You need a very sturdy rear to the ship, though, and a POWERFUL radiation shield. Again, no long distance travel here, but certainly quick travel. And there is a lack of steering control.
Hold on, I know, you're thinking "this is just science fiction nonsense". Well, yes, it is, but Einstein also acknowledged that his theory of relativity left open the possibility for wormholes and scientists have been postulating that they actually exist. If they do exist, then one could possibly use them for travel.
Wormholes can be described simply as bridges through non-real-space--meaning not in 'our' perception of space, but through the fabric of space--connecting x and y. There's no real way to know what a wormhole would look like, since none of us have been in one. Maybe they would be like bright tubes as we've seen on TV, or perhaps you wouldn't even notice you were in one. In any case, you go from here to there.
There are different types of wormhole uses. One is a drive within the ship itself that opens the wormhole. Another is the use of gateways/portals that open the wormholes either permanently or temporarily upon request. Generally speaking, wormholes require negative energy to work, at least according to the hole that Einstein left. And travel would likely be relatively fast depending on the distance you are traveling.
- Solar Power
While this could not really be much use for long distance travel, it's practically free. Once we perfect the technology we can easily build probes and ships that are powered by the sun, making the cost of travel in our solar system cheaper and more accessible. The idea is using solar panels or solar sails to capture the sun's rays and convert it into energy that can be used to propel a ship. It would likely be exceedingly slow going, but it's free.