Some definitions for waves

Wavelength - The distance between 2 crests, or 2 troughs, or, one complete crest and one complete trough. This can shown graphically by plotting the displacement on the vertical axis of graph and the distance on the horizontal axis.

Amplitude - The strength of the signal produced by a wave.  It is represented by the displacement from the horizontal axis.  The greater the amplitude, the stronger the signal, the brighter the light or the louder the sound.

Crest - the part of the wave at the highest point from central point that it will oscillate around.

Trough - the part of the wave at the highest point from central point that it will oscillate around

Period - the time it takes one wave to pass a set point.  Note that if you graph it now, the horizontal axis is now time, not length.  Period is denoted by the symbol T.

Frequency - the number of waves that will pass a set point in one second.  The units to describe this is Hertz (cycles per second).  If you can imagine all the waves in the figure above are travelling at the same speed, you can see the idea with frequency. Frequency is denoted by the symbol f.  

There is a mathematical relationship between frequency and period.  It is as follows:

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