Using the Fourier transform, any signal can be analysed into its frequency components.
Every signal has a frequency spectrum.
We can move back and forth between the time domain and the frequency domain without losing information.
The above statement is true mathematically, but is quite incorrect in any practical sense - since we will lose information due to errors in the calculation, or due to deliberately missing out some information that we can't measure or can't compute. But the basic idea is a good one when visualising time signals and their frequency spectra.
The diagram shows a number of signals and their frequency spectra.
Understanding the relation between time and frequency domains is useful:
For example a sine wave takes a lot of information to define accurately in the time domain: but in the frequency domain we only need three data - the frequency, amplitude and phase.