Many different sites on the internet host streaming video and audio content. Almost none of these sites will let you download the content to your local machine, but that doesn't mean it can't be captured.
Examples of just a few tube sites on the net include: youtube.com, vimeo.com, vevoo.com ; and there are many available regionally, like those operated by networks: ctv.ca, cbc.ca.
MP3 stands for MPEG-1, Layer III (where MPEG = Motion Picture Experts Group). This is the audio portion only of MPEG-1 video. It is a popular format for compressing audio into much smaller files than those recorded on a standard CD of DVD. Thanks to the early adoption the format recieved in the 1990's it is compatible with many devices and thus enjoys incredible popularity. Of course all the compression comes at a price.
To order shrink down to much smaller sizes that its CD equavilent the mp3 file must give up some precious bytes of data. Due to the nature of this tradeoff mp3's will always be of slighty lesser quality than the original. A master of mp3 encoding can knows how to strike the proper balance between a smaller file size and greater audio quality.