plead

ive heard people say this a lot but never cared to ask. so can anyone explain what exactly is murphy's law & why people say it at certain situations? same with "i plead the 5th".. WTF??

Lawyers for pop singer Michael Jackson say the star will plead not guilty to child abuse charges when he appears in court on 30 April after being indicted. http://news.bbc.co.uk/2/hi/entertainment/3648419.stm What do u think? Is he guilty or not?