Cyclotest is what is called a "Sympto-Thermal" monitor. It is a small digital device with a built in thermometer.
Aside from testing your urine every day to see the LH surge before ovulation, your temperature when you wake up is an extremely reliable indicator of when you are about to ovulate. Most fertility charts use temperature, known as BBT.
In addition to BBT, cervical mucus (CM) also changes when you are about to ovulate. It becomes stretchy and sort of like egg white.
With the Cyclotest monitor, you take your temperature each morning when you wake up (there is a built in alarm) and if you notice stretchy CM you press the round button in the middle. The cyclotest displays your level of fertility for that day and the anticipated days. If no baby shows up on the screen, you are not fertile at all, one baby and you are in your fertile 'window' - about 5 days long, and two babies you are highly fertile - that usually lasts about 2 days.
It uses the basic outline of any good fertility awareness/NFP system but does the whole thing automatically. Personally, I found it to be a lifesaver because I am so busy and tired all the time, not a numbers person, and would be scared of making a mistake with imputing all my own numbers. And its not incredibly expensive like some other devices are.
Hope that was clear and helps!