Hellschreiber means "shiny writing" in German. It's an old (1929) German direct printing mode used in World War II. It's inventor was a Dr. Rudolph Hell, which is one more reason to call the mode "Hell" and be through with it.
According to the history here, the original mechanical Hell was a popular alternative to more complex and expensive teleprinters. It was extremely simple. The receiver had two moving parts: a rotating print head and a tape feeder. Copy was printed, in pixels, onto an endless tape of treated paper. There were no carriage returns or line feeds.
The transmitter wasn't a whole lot more complex. A keyboard selected the proper wiper on a rotating drum containing contact strips for all characters. A space was nothing at all - a null frame. The result was a series of simple on-off pulses that could key a 900-Hz audio oscillator (for phone lines or MCW emission), or sometimes straight on-off CW. Today, the audio pulses are sent to a USB transmitter (350HJ2C mode). The narrow bandwidth is achieved through the use of raised-cosine shaping, making the mode less clicky.
This mode obviously lends itself well to computer software, and it is included in most multimode packages. There's also one of those nice stand-alone IZ8BLY programs. All of these use a virtual tape, printing lines of characters across white (or sometimes optionally yellow) strips on the screen.
Hell has the interesting attribute of being completely intended for visual reading by the receive operator. While its creation was and is digital, its reception is fuzzy. This is especially true if the software allows a grey scale, making the mode even fuzzier.
Fuzzy modes are quite the thing right now, since they allow people to do more and better processing than any practical software ever would. Things like "almost" and "that looks right" become a third logic state. SSTV, FAX, and CW Morse telegraphy are also fuzzy modes.
The mode most commonly used today is based on a robust, WWII, military variant named Feld-Hell ("Field Hell" ) You can make jokes like "War is Hell." Everyone else does.
Feld-Hell codes characters into series of on-off pulses, using a simple 7x7 character cell. For better resolution, two half-height pixels are transmitted instead of one full-height pixel. There's no key-up betweeen pixels, so they print together. This is what allows the half-height font smoothing. (It would strongly appear that Dr. Hell had hit upon the idea of sub-pixel fonts.)
This leads to a system where 150 characters are transmitted per minute, with each pulse 8.163 ms long. Effective throughput is 2.5 characters per second at 122.5 baud, or approximately 25 words per minute, which is plenty for hand typing.
Today's tape is a virtual one, drawn on the computer screen, and it's usually white. The audio tone is typically 980 Hz, for zero-axis crossing synchronous with the pixels, also conserving RF bandwidth.
Receiving consists of tuning in the Hell signal until the copy is as clearly black and white as you can get it. Waterfall displays are helpful. The signal sounds like kind of a bzzt bzzt, bzzt bzzt, which can be confused with impulse QRN from sparking electrical equipment, etc.
Weak signals will print lighter in grey scale. As you get down into the noise, the print just becomes harder to read, as opposed to just stopping. While printing can be seriously messed up by the usual HF propagation effects, the human brain can usually sort it out.
QRM is another matter. You wind up with abstract art.
Although characters are inherently in phase, timing errors cause the lines to slope. Computer sound cards are very prone to this. Hellschreiber has traditionally printed the copy
TWICE
TWICE
on the tape, so that one line will always be visible. It's weird, but it works.
A good place to lurk for Hellschreiber is around the amateur calling frequency of 14063 kHz USB. Lesser activity is on 7063, LSB or USB (not important, except for frequency accuracy). Most software also implements Hell in PSK, FSK, and a DX mode, though I haven't heard any of these much. There are also different fonts, which is no small achievement in such a character-based mode. Use of a large font can sometimes improve readability in noisy conditions.