Parts
- Resistors
 - Diodes / LEDs
 - Capacitors
 - Integrated Circuits (IC)
 - Transistors
 - Quartz Crystal
 - Coils & Electromagnets
 - Actuators
 - Switches
 - Encoders
 - Batteries / Solar Cells
 - Motors
 
Sensors
Sensors can be based on:
- Resistance
 - Capacity
 - Frequency
 - Integrated Logic
- Voltage (PWM, Analog-Out)
 - Serial
 - IIC or I2C
 - Digital Logic: TTL & CMOS compared
 
 
Circuits
- Voltage, Current & Resistance
 - Powersources (+,-,GND,Vss,Vdd), moving direction
 - Parallel & series circuits
 - Cryptic abbreviations: TTL, CMOS, I2C/IIC, RS232...
 - Integrated Circuits:
- DIP (Dual Inline Package)
 - SMD (Sub Miniature Devices)
 - Identifying Pin-Numbers
 
 - Prototyping
- Alligator-Clips
 - Breadboard
 - PCBs (Printed Circuit Boards)
 
 - Working with the Multimeter
 - Finding and reading datasheets
 
general
- Units and their prefixes (pico, nano, micro...)
 - Frequency and Sinewaves
 - Digital numbers representation
 
Links
All About Circuits: online electronics book
http://www.loullingen.lu/unterrichtsunterlagen.php electronics course from Claude Loullingen
http://www.elexs.de/kap2.htm
http://www.ecs.umass.edu/ece/m5/tutorials/tutorials.html
http://iad.projects.zhdk.ch/physicalcomputing/
Electronics and Electronics related Tutorials on sparkfun.com 
Simulation with visualization of current flow (java applet)
...and of course the suppliers page…