It has an input register that is 8 decimal places wide, an output register of 11 digits, and a counter register of 6. When the crank is rotated once, the contents of the input register are added to the output register, and the counter register is incremented. If the crank is pulled out when it is rotated, the input register is subtracted from the output register, and the counter register is decremented. Finally (hang on to your hats, folks!) the top half of the cylinder may be rotated such that the input register's digits are applied to whichever place setting in the output register you want, with the corresponding counter digit being incremented. That last feature totally rules, and places this thing in a whole 'nuther category of calculator.
Babbage would swoon at the cylinder-rotation feature. It provides for a fast multiply-by-ten, and can be used to make repeated addition feasible for multiplication of large numbers. To do 6,357 times 32, for example, you set 6357 in the input register and spin the crank twice. The output register now contains 12714 and the counter register contains 2. Then, you lift the top half of the cylinder up and shift it over one place so that the 7 in the input register's one's place is underneath the ten's place in the output register, and you turn the crank three more times. This effectively adds 30 times 6357 to the 12714 that's already in the output register. The output register reads 203424 and the counter register reads 32, allowing you to verify that you turned the crank the number of times that you intended to. Multiplying 6,357 by 4,872,234 is pretty easy too, but you want to put the larger number into the input register and crank the 6357 in. It would take 22 turns and three shifts. I did this and got 30,972,791,538 as the answer, which the ten digit display of my TI-34 had to round up. Rest assured that the TI-34 is now landfill.
The Curta was designed by Curt Herzstark while he was a prisoner in a Nazi concentration camp during World War II. Thankfully, he survived, and after the war he took his amazing design to Lichtenstein where it was manufactured by Contina AG Mauren.
A larger model, the Model II was also manufactured by Contina AG Mauren. It had an input register 11 places wide, 15 output places, and a counter 8 places wide. Both models came with an instruction booklet that stepped a user through a variety of arithmetic problems, including derivation of roots and a few accounting and trig problems.
My mom discovered this Curta in a Salvation Army junk store in Everett, Washington. She undoubtedly got it because she knew I'd be drooling all over it and needed to be taught a lesson - I was always ranting about how boring junk stores are and how much I hated being dragged out to them when I was a kid. Okay Mom, you rule. It was an awesome find.