"Hz" represents "Hertz," which is a measure of sample rate or audio frequency. The average human ear, in its prime, can typically hear frequencies between 20 and 20,000Hz. The difference between 44,100Hz and 48,000Hz is more of an issue of quality, rather than something our ears can normally differentiate.
44,100Hz represents the sample rate, or frequency, of a digital audio file. This is the standard for audio CDs, as it is slightly more than double the sample rate of analog sound, which typically maxes out at around 20,000Hz.
48,000Hz also represents the sample rate, or frequency, of a digital audio file. It is the standard for digital video files, as well as digital audio tapes (DATs).
These frequencies are basically a measure of their quality. Ironically, it is a quality that cannot truly be appreciated by the human ear, but some argue that it can be felt, and that the overall clarity can be detected. The highest standard sampling rates for digital audio have reached 192,000Hz, and the limits will continue to be pushed.