Time domain and Frequency domain
Both domains describe the same thing, but seen from different perspectives.  I like to think of the frequency domain as the time domain skewed through 90°.

In the time domain you have your typical signal waveform - the X axis is time passing, and the Y axis is the level of the signal at any one point in time.

Now, a signal is made up of lots of frequencies.  Take, for example, a "simple" square wave.  As time passes it alternates between a low and a high level.  Very simple.  However, fundamentally, there is only the sine wave, so that square wave must be made up of multiple different sine waves added together.

The frequency domain is a snapshot of a brief moment in time, and describes what frequencies are present in the signal during that time.  A square wave may be fundamentally 1KHz, but in the frequency domain we can see that it is made up of sine waves at 1KHz (the fundamental frequency), 3KHz, 5KHz, 7KHz, etc - each one smaller than the other.

Another good way of looking at the relationship between the two domains is to think about frequency and period.  In the frequency domain a sine wave may be 1KHz, but in the time domain it has a period of 1ms.  They both describe the same thing, but looked at from different directions.