UGC 7321 is a spiral galaxy at a distance of about 75 million light years (23 Mpc) [1] that is seen edge-on from Earth in the constellation Coma Berenices. In the case of many other spiral galaxies that we see edge-on, the disk is either tilted just a little, or the disk looks a bit puffy, so the galaxy still looks rather thick despite the fact that we're looking at the edge of a disk. Some spiral galaxies that we see edge-on from Earth may even look a bit warped. UGC 7321, however, doesn't have a puffy-looking disk. It's disk looks relatively skinny. Additionally, while many other spiral galaxies have puffy spherical bulges of stars in their center, UGC 7321 has no such bulge. It simply looks flat like a pancake, sort of like the stereotype of Kansas.
Astronomers have zeroed in on the skinny appearance of UGC 7321 and they have labelled it as a superthin galaxy. Now, UGC 7321 is not the only superthin galaxy that astronomers know about, but it seems to be one of the closer superthin galaxies, which means that if some astronomer wants to understand these galaxies, they will go look at UGC 7321 first.
To understand why UGC 7321 is superthin, it's easier for me to explain why other galaxies are not superthin, and this mainly involves gravitational interactions among galaxies. Two spiral galaxies that pass by each other will gravitationally distort each other's disks, possibly warping them or making them look elongated, and it is even likely that the two galaxies will merge together. If two spiral galaxies of the same size merge together, their orbits will end up completely scrambled, with the stars moving in random directions. Once the merger process is complete, the resulting object will be an elliptical galaxy. However, when spiral galaxies of different sizes merge together or when a smaller dwarf galaxy falls into a larger spiral galaxy, the disk of the larger galaxy will largely remain intact but may end up looking warped or puffier, while all of the orbits of the stars in the infalling galaxy could end up being scrambled, and they would end up moving in random directions. Many of the smaller galaxy's stars could end up in the larger galaxy's bulge.
However, none of this seems to have happened in UGC 7321. It's just a flat, rotating disk with no bulge. The gas in UGC 7321's outer disk, which is more extended than the stars in the disk, looks a little warped, indicating that the galaxy might have had some sort of gravitational interaction with a smaller galaxy at some point [2], although that's not really certain [3]. For the most part, though, it looks like UGC 7321 has managed to avoid any type of interaction with any other galaxy.
This is because UGC 7321 is in a really isolated location in space [4, 5]. Many other galaxies, including the Milky Way, are in gravitationally-bound groups or clusters of galaxies, and many larger spiral and elliptical galaxies, including the Milky Way, have lots of satellite galaxies. UGC 7321, however, is completely isolated. It doesn't even seem to have any dwarf galaxies orbiting it [6]. So, with no other galaxies nearby that could distort its shape, UGC 7321 is able to exist by itself as an unchanged, undistorted, undisturbed disk.
When other galaxies gravitationally interact or when they merge together, interstellar gas clouds within the galaxies either collide or otherwise gravitationally collapse. These collapsing clouds will form lots of new stars, and sometimes galaxies merging together will trigger events called starbursts where stars form at rates that are factors of 10 higher than what is typically seen in ordinary galaxies. However, because UGC 7321 is so isolated, it's never undergone any type of starburst event. Stars still form in the galaxy, but it somehow seems underdeveloped [7]. This demonstrates how important such interactions between galaxies are for forming new stars, sort of like how children interacting at school are more likely to develop better social skills than children who stay at home or how adults interacting at work are more likely to get creative new ideas than people who work remotely. (I have to admit that I do a lot of remote working. Maybe I haven't had any creative new ideas lately. I should try experimenting with my smoothie maker.)
Anyway, to summarize, UGC 7321 is a rather amazingly flat spiral galaxy that astronomers are really interested in because it provides very useful information on the processes other than gravitational interactions between galaxies that shape and change galaxies in general. It's been a rather intriguing object for astronomy studies for decades, and it is going to continue to be targeted by astronomers for decades to come.