A capacitor stores electric charge. It can be used with a resistor in a timing circuit, for smoothing a supply (it provides a reservoir of charge) and can be used as a filter (blocking DC signals but passing AC signals). Unpolarised capacitors usually have small values, less than 1µF.
Oscilloscope An oscilloscope is used to display the shape of electrical signals - showing how they vary with time. It can be used to measure voltage and time periods.
Diode A device which allows current to flow in only one direction.