(These are excerpts from Prof. James Ama's standard work "Riding the Rag. An introduction to jumpspace physics for non-physicians", Core Books Press, Capital/Core 1102.)
What is jump space?
Jump space is a phenomenon separate from normal space, but related to normal space in a specific manner: It seems that the masses of the galaxy's stars form an extrauniversal structure, which is at average just 0.3 LY thick, but of unknown width and lenght when speaking of its corresponding expanse in normal space. For simplicity, this corresponding expanse will be called "jump space structure" or short "JSS" in the following. The foreign universe where the actual transport takes place will be called "jump space universe".
Like a crumpled table cover, the JSS touches almost every star system in Charted Space. It is currently believed that the creation or existence of both the JSS and the jump space universe are in some as-yet-undetermined way related to the gravitational influence of the stars and the galaxy.
Why are jump maps flat?
Jump maps show the "crumpled table cloth", the jump space structure, in a way that easily shows the jump distances in relation to each other. It is an irony of universe itself that one of the most complex concepts in physics can be shown and exploited on a simple piece of paper just by smoothing down any model of the jump space structure.
How does a jump drive work?
A jump drive is, to put it simply, an extremely complex device which drops anything within its supporting jump grid structure into the jump space universe, provided this grid is within the JSS (the crumpled table cloth structure in realspace) and at least approximately 100 diameters from any large masses. Full stop. That's all. The several things that happen to the ship after it enters the jump space universe is not due to the jump drive, but is merely an effect of the jump space universe itself.
So what exactly happens to a ship that "jumps"?
First, the jump drive is engaged. In this process, a lot of matter in its simplest form (i.e., hydrogen) is needed to transfer into jump space universe. The higher the volume of the ship, the higher the amount of hydrogen needed. In addition, a significant amount of energy is needed to in effect 'open a hole' between normal space and the jump space universe.
Second, the ship is transferred into jump space, emmitting a jump signature which shows the occasional watcher either the direction or the distance of jump along jump space, but never both. The ship is now in the jump space universe.
Third, the jump space universe transports the ship along the predestined path at an incredible speed. (This speed is usually claimed to be in 'parsecs per week. See the section on jump distances for more information.).
Fourth, the jump space universe does something very practical for the space traveller: It spits out the ship after a certain period of time, which is 170 hours plus or minus 10 per cent. This is believed to be due to the less-understood ability of the jump space universe to "get rid of universe-foreign matter". Without this, a jump drive would be useless. (see below, What can a jump drive not do?)
Finally, the ship re-enters real space, again showing a jump signature, this time showing the direction where or distance from where the ship comes from. The jump is complete: by travelling through jump space universe, the ship has passed a vast distance within a single week.
A jump drive simply transfers a ship into the jump space universe, consuming a lot of hydrogen and energy in the process. That means, it cannot re-transfer the ship from jump space universe back to real space. Once you have entered jump, you have to wait until the universes rearrange their affairs.
The jump space universe has, to put it simply, several "layers". There is an unknown but very large number of these layers, each corresponding to a different speed of travel. Most of these layers are, however, worthless for travel, since their speed is much too low to be economical. The highest possible speed that the jump space universe offers within the limits of our current technology is six "jump distances" on a jump map. Note that reaching the deeper layers of jump space universe requires even more hydrogen and an even more complex jump drive, based on our current understanding.
The actual length of the "jump distance" is dependent on the structure of the jump space universe, which seems to be far from being homogenous. The actual distance travelled is therefore dependent of the location where you travel. In most places, the actual length of a jump distance is between approximately 2.7 and 3.6 light years. There are exceptions to this rule, however, where the differences between "jump distance" and one Old Terran Parsec (that is, 3.26 light years in real space) are quite large.
The fact that a "jump distance" is generally called a "parsec" is a matter of history: Two jump distances equal about 2 parsecs (6.6 LY) between Sol and Barnard's Star if you make one jump-2 (or two jumps-1 etc.) there (and only there). When the Solomani encountered and later conquered the Vilani empire, they substituted the old Vilani expression for "one jump distance" with an expression they were familiar with: Parsec.