Audio files are probably the most downloaded files. Most of the files are 'MP3' files. Another type of file that's used on computers is the .wav file. Wav files are essentially the same as the raw, unencoded files that you have on a CD. Wav files are typically much larger than MP3 files so they are not as popular for downloading.
MP3 files are 'compressed' audio files. The compression allows the file to be smaller with little or no loss in sound quality (depending on the compression level). The compression level is given as a 'bit rate'. The 'bit rate' is the number of pieces of information (think of the bits as the letters of the words in a book) that will be processed per second. For the relatively standard bit rate of 128kbps (128 thousand bits per second), the processor is essentially reading 128 thousand letters for every second. If you increase the bit rate, the quality will get a little better sound quality but 128kbps is already pretty high quality (very close to CD quality) so you don't really get much of benefit if you go higher (especially when listening on inexpensive headphones or low-end computer speakers). The highest bit rate commonly used is 320kbps. The lowest used for music is generally 128kbps. For something like recording the audio from a telephone, you may use something as low as 8kbps. The overall file size is directly related to the bit rate multiplied times the length of the recording (in seconds).
A little more on compression... When an MP3 file is converted from a wav file to an MP3 file, the process is called 'encoding'. It's essentially converting an analog file to a digital file (although the analog file may be stored digitally). During the conversion, the encoding software does a couple of things. It will remove all of the things that are not audible. It will also find the things that are common to both channels. When it encodes the audio, it will not include the parts that were not audible and it will only keep one copy of the components that were in BOTH channels. When the audio is played back, the information that was common to both channels will be placed in both channels. The things that were not common to both channels will be reproduced in there respective channels. All of this together helps to make the file smaller. For 128kbps audio, the files are typically 1/10 the of the same file on audio CD. For example, a song that, in wav format is 50 megs (megabytes) will be 5 megs or less in MP3 format when encoded at 128kbps. If you were to try to download the wav file, it would take 10 times longer.
In digital electronics, the information is a series of zeros and ones (nothing else). To express numbers greater than 1, the zeros and ones are used in digital words. The individual letters of these words are known as bIts. The following 8 digit word, 00000101 is actually the number five. 8 digit words are called bYtes. Bits are the individual letters and bytes are 8 digit words. Typically, for data transfer or data streaming, the data rates are specified in bits (each letter passing through is counted). For data storage, bytes are typically used. When looking at certain numbers like 175kb, you may not know if they indicate bits or bytes. If the number is properly written, it's easy to tell. If the 'b' is an upper-case b, then it's bytes. if the b is lower case, then the number is bits.
In the bit rates or in storage, you will see a prefix of giga, mega or kilo. Look at these numbers simply as multipliers. If the number is expressed in kilobits, then the number is multiplied by 1000. For example, if a number like 120kb is given, this means that the actual number is 120, 000. If the number is 120Mb, then the number is actually 120,000,000. If the number is 120Gb, then the actual number is 120,000,000,000.