The SI unit of electrical resistance (its symbol is Ω - which is the Greek letter 'omega' - an obvious choice!). Whenever an electrical current (electrons) flows through a conductor it experiences some form of 'resistance' to its motion. This resistance is the property of the material which makes it a good or bad conductor. An electrical component is said to have a resistance of 1 ohm when a current of 1 amp causes the voltage difference across its terminals to be 1 volt. That’s a long-winded way of saying:
1 ohm = 1 volt / 1 amp
Long wires have more atoms/molecules in them than short wires. Therefore the electrons/ions experience more 'collisions' as they flow through them so more energy is transformed to heat. In short, the resistance has increased.
'Thicker' wires have a greater cross sectional area, therefore more electrons can pass a particular point every second. This results in a greater current. Hence the resistance is reduced.
'Hot' wires contain more energetic atoms/molecules. This restricts the flow of electrons/ions, increasing the resistance and decreasing the current.
The easy way to remember Ohm's Law:
In the triangle diagram (below), cover up the quantity you want to find and the diagram then shows you how to find that quantity when you know the other two values. So, for example, if you know the current ( I ) and the voltage ( V ) and you're asked to calculate the resistance, cover the R and you're left with V above the horizontal dividing line and I below it, so:
R = V / I.