When an electronic circuit is created often times precise amounts of resistance are needed. This is often achieved by adding components called resistors. These resistors have two connections which are used to hook into a circuit to create the resistance required.
Inside the resistor is a coil of copper wire with a ceramic rod running through the middle. The number of turns the copper wire has dictates how much resistance the resistor offers. Copper resistors are generally more precise and stable than their carbon-film counterparts.
A common type of resistor used is the high voltage resistor, these are used in many common appliances and equipment. Uses such as printers, copiers and air conditioning units. As well as this high voltage resistors are used in machinery such as wind turbines and various medical equipment.
Variable resistors are another common type of resistor available today. These, as the name suggests, are adjustable based on the level of resistance needed. The potentiometer is the most common type of variable resistor available to buy today. A popular use of a variable resistor is for volume control on an amplifier and the tuning of circuits.
Digital resistors are a type of variable resistor where the resistance is performed by digital signals rather than mechanical movement. A protocol called I2C is usually used to change the resistance in small steps, simple up/down signals can also be used.
These are just some of the resistors available on the market today. More specialist resistors can be purchased for specific requirements such as the high voltage resistors mentioned above as well as high temperature and military grade resistors.