Some resistors are designed to change in value when heated. Some resistors change in value when exposed to light. Most resistors are colour coded to indicate their value and tolerance. Wire wound resistors have their value written on them. Resistors generate heat.
Resistors have a wattage rating.
The higher this rating the more heat they can dissipate. To limit the range of resistor values to a manageable number a preferred range only is available. 3.3M would be written as 3M3 and 1.8k as 1K8 etc. On circuit diagrams tolerance is indicated by the following letters. These can be operated by means of a knob on the control panel.