What are the negative consequences of downloading music off the internet in regards to legal matters and and technical matters?

Technical Matters: Only negative consequences are A: slight possibility of virus or other malware being included with download. This is rare however, and you should be using anti-virus and anti-spyware software anyway. B: Audio will be of lower quality than physical CD unless is in FLAC format, which is relatively slow to download and quality level may vary substantially dependent on quality of source, which is often unstated. For example, a FLAC copy of a low bitrate mp3 will not sound better than the mp3 does. Though this seems obvious it seems to escape some people. Vast majority of non-iTunes downloads are in mp3 format. Standard quality measure is bitrate (number of bits processed per second). What quality mp3 is acceptable depends on personal preference and intended use. For example you would normally want a better quality (ie higher bitrate) mp3 for playback on an expensive home stereo than for playback on small computer speakers. Most people apparently don't consider the difference significant for pop music at bitrates of 192k or higher when played on today's standard sort of device such as an iPod with stock earbuds or a boombox or small computer speakers.

Legal Matters: Assuming you are in USA: Very complex question to answer, individual liability currently still being defined by courts. From a current legal perspective downloading copyrighted music without permission is a form of illegal copying that is not covered by fair use, because 1. you are not time-shifting a continuous stream and said copy is intended for repeated use, so TV videotaping precedent does not apply; and 2. primary intention is not format conversion or archiving, so cassette precedent does not apply. In reality there will probably be no negative legal consequences, but there is a slight risk you could be targeted for civil action by the RIAA. As of 1-1-08 afaik no individual downloaders have been sued, all defendants are accused of uploading at least some significant number of songs. So if you do not participate in a p2p by uploading songs you are extremely unlikely to be targeted, however there is still some slight risk.

If current trends continue, medium-quality mp3's without DRM will become available for free or very low cost from legal sources; thus the need to take even the slight risk of being targeted by the RIAA is expected to gradually dissipate.