Standards:
When floppy drives were origionally made available the only disk size available was the 8" floppy. The drives that read these disk spun at 360 RPM, had 77 tracks, and talked to the controller over a 50 pin cable at either 250Kbps (SD) or 500Kbps (DD). These drives were fairly expensive and weren't commonly seen in the home computer market. In the late '70, Shugart and Associates interduced the 5.25" "mini-floppy" disk drive which was basicly a less expensive and more affordable version of the 8" drive targeted to the home computer market. The first 5.25" floppy drive was the Shugard Assoc. SA400, which is a single sided 35 track drive. The 34 pin interface used for this drive is the same one used over the years and which is currently used today. Some companies, notable Apple and early Commodore, chose to use a propritary drive while most others stuck with the standard (making replacement and upgrades much easier). They also chose the 300 RPM speed for the drive, making it a little slower than the 8" drives before it.
Density, single vs. double:
One source of confusion is the difference between single and double density. Single density is fairly uncommon any more so it's understandable that many computer users have never even used a machine that uses single density storage. The difference is the choice of FM (Frequency Modulation, for SD) vs. MFM (Modified Frequency Modulation, for DD). Without going into the details, the difference is in how the change in magnetism on the media is related to a 1 or 0 by the controller. In other words, the disk and drive are not bound to any one density, it's determined by the controller only. The only notable exception is that some media may fail when used for double density storage, as it is more damanding (it does store twice the data after all).
Density, low vs. high:
Unfortunatly using the term "destity" again as was done for single and double density just makes matters more confusing, as the two are totally unrelated. In fact, there's no reason we couldn't have any combination of technology (ie a single density high density disk), but high density is a few years newer than double density, so typically all high density disks are double denisty (MFM incoding) as well. The main difference with high density drives vs low density is that they use a different media that is able to fit roughly twice the bits on a track. In order to communicate this additional data to the computer the controller must now talk twice as fast as before, at 500Kbps. Again, to make things more confusing, there are two variations between drive types. 5.25" high density drives typically spin at 360 RPM where all other 5.25" and 3.5" drives are 300 RPM. This has some interesting advantages and disadvantages. A 5.25" high density drive spins the same speed as a 8" drive, and can now handle the same data rate, so aside from the different data connector, it's a direct replacement for an 8" drive. Unfortunatly, this also means that the drive spins to fast to allow it to work in place of a low density drive. Usually a 5.25" HD drive always runs at 360 RPM and uses a 300Kbps data rate in low density mode, though some are jumperable such that they are 360 RMP for HD, 300 RPM for LD. On the other hand 3.5" drives are 300 RPM. This is why 3.5" HD drives can hold a little more data than a 5.25" HD drive, they spin slower yet talk at the same data rate, so they're able to read/write a little more data with each revolution of the disk.