I have a Mazda - 323 S V (BA) - 1.8 i 16V (114 Hp) and the data sheet for it says:
Fuel consumption (economy) - combined 7.9 l/100 km.
How is this calculated and under what driving conditions?
This is an average of the figures for constant speed (highway) driving in top gear and city/traffic driving with lots of gear shifts. The ratio is usually 50/50 so they'll measure e.g. 100 miles of highway driving, then 100miles of city driving, add the figures together and divide by two to get the combined average.
Though to get those same figures, you're going to have to be niggardly with the accelerator pedal. I had a car with a claimed highway consumption figure of 8l/100km and 15l/100km city figure. I managed to improve those figures slightly, but I never went above 2000RPM and I coasted as much as possible and never sped up unless absolutely necessary. If I even momentarily had some fun with the car, the figures went up to 12l/100km and 22l/100km respectively.
So basically the combined figures is the theoretical best figure you'll get if you drive like your grandmother. Though if you drive like a normal person, you can probably add about 20% onto the claimed figure.