Speaking non-technically, a time domain graph shows how a signal changes over time, whereas a frequency domain graph shows how much of the signal lies within each given infinitesimal frequency band over a range of frequencies. A spectrum analyser on a stereo is an example of a frequency domain graph.
If a signal is known in the time domain, a frequency domain graph can be constructed using a technique known as a Fourier transform. This technique relies on Fourier series.