Just FYI...
The following is "typical", without seeing your actual panel.
The markings on a breaker indicate the maximum current the breaker will allow before opening ("breaking") the circuit. The breakers marked 15 will pass 15 amperes, those marked 30 allow 30A, etc.
If you had a main breaker, its rating would be larger than the largest breaker in your panel, but probably smaller than the sum of all the breakers. If I had to guess, I suspect your main breaker (in the utility room) is between 100A and 150A.
The twelve (six sets of 2) 15 Amp breakers provide 120V power to things like outlets, light fixtures, and medium power appliances, such as refrigerators, dishwashers, washing machines, etc. The 2 breakers in a single breaker slot is called a tandem breaker, and it's used simply to save space... the two breakers control different branch circuits.
The 30A and 40A breakers are a different story. They probably look like two single breakers side by side, with a bar connecting the two handles. Opening one breaker also opens the other. These are used for 240V circuits, which have two "hot" conductors (120V circuits have a single hot). 240V power is used for higher wattage devices, such as hot water heaters, air-conditioners, ranges, ovens, heat pumps, etc.
If there isn't a legend inside the breaker panel as to which breaker controls which circuit, a rainy Saturday project would be to turn off one breaker at a time, then see which circuits go out. Make up a chart showing what each breaker does, and post in it/near your breaker panel.