Different valve types and pump heads
Inner tubes usually have one of two types of valve: Presta or Schrader. Schrader valves are the same as those found on cars and are usually seen on mountain, hybrid, and kids bikes. Presta valves are typically seen on road bikes and are longer and thinner than Schrader valves.
Because of this, pumps have to have different types of heads in order to work with the different valves; a pump for a Schrader valve won't work with a Presta valve and vice versa. However, many pumps now feature twin heads that will fit both, and even smart heads which will automatically adjust to the different valves. These pumps are ideal if you have a range of tyres with different valves that you need to keep topped up.