Analog Video
All analog video standards are designed around specific frequencies. The higher the frequency used, the more horizontal detail can be represented. Video signals are rated in MegaHertz (MHz), each of which is equivalent to 1 Million Hertz. PAL's 5MHz signal equals 5 Million waveforms per second. While this sounds like a lot, once you divide it between 576 lines on each of 25 frames it's not really that much detail.
Audio Frequencies
Sound waves used in standard audio equipment range from 20Hz through 20 kiloHertz (kHz), which is equivalent to 20 Thousand Hz. While there are endless debates around whether these frequencies are actually the limits of human hearing, they're currently the standard values used in both content and playback/recording equipment.
Sampling Frequencies
The primary importance to understanding frequencies for most people is the use of sampling. When analog video or audio is stored digitally it must be sampled. Sampling replaces the continuous line of an analog wave with points (samples) at regular points in time. According to the Nyquist-Shannon Sampling Theorem, the frequency of the samples must be slightly higher than twice the frequency of the original wave.
In the examples above, in order to accurately sample a PAL video signal you'd need to use a frequency of more than 10MHz, which roughly equates to a Capture Resolution of 520x576, meaning a Capture card capable of 640x576 or higher would be suitable. For sampling 20kHz audio you'd need a sample frequency of better than 40kHz. Audio CDs are sampled at 44.1kHz. In reality it's best to get as many samples as possible because actual consumer equipment isn't capable of the precision required for correct Nyquist-Shannon sampling, and is therefore prone to errors. Those errors are minimized if the sampling frequency (Resolution for video) is kept as high as possible.
Related Guides
Digital Video Fundamentals - Resolution and Aspect Ratio
