The sounds of vowels are determined by the combination of the vibration of the vocal folds and the effective length of the vocal tract. The vibration of the vocal folds (voicing) gives all vowels a quasi-periodic waveform in the time domain with harmonics that are viewable in the frequency representation of the signal. The space between these harmonics in the frequency domain, or the reciprocal of the length of one period in the time domain, is the pitch (or fundamental) frequency of the voice. In the spectrogram plots of each word the harmonics of voiced phonemes are seen as either horizontal (narrowband) or vertical (wideband) striations. Note that not all voiced phonemes are vowels but that all vowels are voiced.
The spectral envelope of each vowel is shaped by the vocal tract which has many different configurations--the three main ones being front, center and back. These locations correspond to the position of the tongue on the palate which determines the effective length of the vocal tract thus creating resonances. These resonances (also known as formants) are the poles of the vocal tract and are one of the main contributors to the shape of the spectral envelope. These formants can be seen as peaks in the smoothed versions of the spectral envelopes that are plotted in red on the frequency plots linked to from the main page. The peaks are especially viewable in the /@/ phoneme.