The flow of charge through any material will encounter an opposing force similar to mechanical friction.  The opposition in the material which converts electrical energy to heat is called the resistance of the material.  The unit of measurement of resistance is the ohm for which the Greek letter omega is the symbol: Ω

Temperature can have a significant effect on the resistance of conductors, semiconductors, and insulators.  For good conductors (like silver, copper, gold, and aluminum), an increase in temperature will result in an increase in resistance.  For poorer conductors or semiconductors, an increase in temperature will in a decrease in resistance.

The schematic symbol for a resistor is:

In the above schematic symbol, the text "R37" is called the reference designator.  That has nothing to do with the resistor per se.  Rather, it is simply a designation assigned by the circuit designer to uniquely identify this specific resistor from all the others in the design.  The number is arbitrary.  The text "100K" refers to the ohmic value of the resistor.  In specifying resistance "K" means to multiply the stated value by 1,000 to get the resistance in ohms (Ω).  So a "100K" resistor is really 100 * 1,000 ohms, or 100,000Ω.  With larger value resistors, it is inconvenient to write so many digits.  By abbreviating a multiplier  "K" for 1,000 one can write resistors with large values using fewer digits.

We may as well visit all the ways resistor values are depicted in schematics!  For values less than 1,000 ohms it is customary (in the U.S.) to simply write the value and ohms (Ω) is assumed.  A 100 ohm resistor be shown on a schematic simply as "100".