A converter changes the voltage level and an adapter only allows one plug configuration to adapt (fit) another. An adapter is a passive device. It does not change the characteristics of the power; i.e., what goes in goes out. If there is 220 volts on the input, there will be 220 volts on the output.
In the USA, the voltage we use is 120VAC at 60Hz. Other parts of the world use other voltages. Europe, for example, uses 220 VAC at 50Hz. Some devices such as iPhone chargers, are designed to use any voltage between 100 and 220 volts at 50 or 60Hz. Some devices have a switch that allows you to use either 120 or 220 volts. If your device is designed to use 120 volts only and you want to use it in Europe, you will need a power converter (transformer) to change the 220 volts from the electrical outlet to 120 volts. If you do not, your 120 volt device will catastrophically fail. In addition, if you are going to power a motor, the power frequency is an important factor. If a motor is designed for 60Hz operation and you apply a 50Hz voltage, the motor's windings could overheat and short out or burn open. However, a motor designed for 50Hz operation will work well at 60Hz.
There is one other consideration in choosing a converter; the electrical load it can handle. Some devices have a current rating (Amps) and others have a power rating (Watts) on their labeling. The total current or power rating of the converter must not be exceeded or the power converter will fail. Add up the maximum ratings of all the devices you will connect to the converter at one time and that will tell you the minimum rating of the converter you should buy.