Limewire isn't a site. It's a program which enables your PC to connect to a file-sharing network called Gnutella (in the same way that Internet Explorer enables your PC to connect to the worldwide web). While you're connected, your PC can copy files directly from other PCs connected to Gnutella (whether by Limewire or another program, such as BearShare).
Similarly, other users can copy files, which have been stored by Limewire, from your PC. It's this fact which can have significant legal consequences.
In most cases,
downloading copyrighted files is only a
civil offence. i.e. you can be sued for it (possibly for many thousands of pounds) but you can't be fined and you can't be sent to prison.
Downloading only becomes a
criminal offence if it is done 'to such an extent as to affect prejudicially the owner of the copyright' or if it's done for the purpose of 'sale', 'hire' or otherwise 'in the course of a business'. So, someone who downloads thousands of files or who sells the files, can be prosecuted under criminal law and face fines or imprisonment.
Remember though, that Limewire makes files on your PC available to other users. This is
uploading or, in the eyes of the law,
'distribution' which is also a
criminal offence and, consequently, can lead to fines or imprisonment.
Section 107 of the Copyright, Designs and Patents Act 1988:
http://www.opsi.gov.uk/acts/acts1988/Ukpga_198 80048_en_7.htm#mdiv107
Chris