A crest is the highest point of a wave within a wave-cycle. A trough is the opposite of a crest, so it is the lowest point of a wave in a wave-cycle.
On a wave at sea, the crest of a wave is a point where the wave is at its highest. A trough is the opposite of a crest, so a trough is a point where the wave is at its lowest.
In the case of an electromagnetic wave which has an electric and a magnetic axis, a crest on either axis refers to maximum oscillation in the positive direction whilst a trough refers to minimum oscillation in the negative direction.
Wavelength refers to a complete wave-cycle from one crest to the next, or one trough to the next.
Frequency refers to the number of complete waves that pass a given point in a given amount of time.
The amplitude of a wave is a measurement of the distance from the centre line (or the still position) to the top of a crest or to the bottom of a corresponding trough.
Amplitude is related to the energy a wave carries. The energy a wave carries is related to frequency and amplitude. The higher the frequency, the more energy, and the higher the amplitude, the more energy.