The movie above shows a simple simulation of the formation of a single tidal stream as a small galaxy (we'll call it "Tiny") merges with a large one (that we'll call "Bertha"). Here is a key to understanding the movie:
 Okay, okay, I know you want to know what that is in light years, but trust me---it won't help with a sense of scale. 1 kpc = 1000 pc = 3260 light years (or so). Now what are you going to do? The point is to compare to things you know the size of already - in this case, the diameter of our galaxy and the position of the Sun in it are good yardsticks. If those things were more precisely defined, astronomers would measure distances in units of the solar distance and/or the milky way's diameter instead of any of these units (actually, they already do anyway, even though those things aren't that well measured!). The only reason we use parsecs is because of how some astronomical distances are measured.
 If Bertha weren't invisible, you wouldn't be able to see the stream formed by Tiny except perhaps at Bertha's edges---Tiny contains far fewer stars than Bertha (in this case, several thousand times fewer!), and they are spread quite thinly over time as Bertha's tides rip Tiny apart, so the stream of stars from Tiny is usually fainter than the background from Bertha.
 Actually, the white dots are simulation particles that are used to track approximately what the stars in the region right around a particular dot location would do. Each white dot has a mass equivalent to about ten thousand stars, so you can think of each one as tracking the collective behavior of that many stars. We do this because to simulate each star in Tiny with one particle would take much longer (more than 10,000 times longer!), and isn't that much more informative in this particular case because we've made so many other simplifying assumptions.
 Actually two arcs: one made of stars that orbit around Bertha faster than Tiny's main bulk (the "leading arm") and one of stars that orbit slower (the "trailing arm").