The L293D is a motor driver IC, popularly called a H-Bridge IC. To find more information about it google "driving DC motors with L293d"(without the quotes).
The 20K preset is a variable resistor, popularly called potentiometer and pot. It is basically a resistor whose value can be varied from 0 ohms to the value of the preset. So if you have a 20K preset you can change its value from 0 ohms to 20K.