They are fundamentally related in that
one cannot exist without the other.
FREQUENCY
In a general context, frequency is the number
of cycles per second. It could refer to an oscillating
pendulum, tidal waves, audio or radio signals.
Whatever the frequency, medium or shape, it cannot
exist without at least two amplitude levels. A square
wave would be an example of this. Triangular and
sine waves would could cycle through an almost
infinate range of values.
A frequency does NOT have to be AC.
Think Digital Square Wave computer clock.
AMPLITUDE
For a frequency to to exist there must be
some change in value over the cycle.
Consider an electrical frequency.
For the usual Sine Wave the +peak would
be as positive above 0v or ground, as the -peak
would be as negative relative to the 0v or Ground.
The values of either of these peaks, would be considered
the amplitude of the wave. Here Vp = Vp-p / 2
Seeing the wave as a graph, or scope display,
the Horizontal scale INDIRECTLY gives the Frequency,
as it can be determined from the cycle time or period
of the graph. So, f in Hz = 1 / cycle time in secs.
From the Vertical scale, the amplitude of the wave can
be determined. However its easier to determine the Vp-p,
and divide it by two to get Vp which is the AMPLITUDE
Relating the Frequency and Amplitude of a signal,
we could say for example,
15kHz sine wave, with an amplitude of 2.6V.