This resistor calculator allows you to calculate total resistance created by several resistors connected in series or in parallel. If you know Physics, you'd be familiar with the concept of breaking complex circuits into simple series and parallel circuits for total resistance. Here, you will also be able to find a calculator that is used for estimating resistance of conductors depending on their size and the conductivity.
It is, basically, an electrical component which regulates or limits flow of electric current in the electronic circuits. A common use of resistors is to offer particular voltage to some active device like transistor.
Keeping everything equal, in a DC circuit, current passing through resistor tends to be inversely proportional to resistance while it is directly proportional to voltage running across it. That's called Ohm's Law. When it comes to AC circuits, the rule applies there as well if resistor doesn't contain any capacitance or inductance.
There can be different possible ways for fabricating resistors. The type used most commonly in the electronic devices is carbon-composition resistor. Graphite i.e. finely granulated carbon and clay are mixed together and allowed to harden. Resistance of this resistor depends on carbon-to-clay proportion. The higher the ratio is, the lower its resistance will be.
Here are the equations used by this resistor calculator for parallel and series resistors. The last one is used for finding out the resistance of conductors.