I first heard about this in Eminem’s Like Toy Soldiers in a line:
… It was crazy, this shit went way beyond some Jay-z and Nas shit.
Can anybody help me understanding what this feud was all about?
It all started when Nas was supposed to appear on Jay-Z's debut album. Jay-Z had used a sample from one of Nas' song on his cut, "Dead Presidents", and asked Nas to come in and lay down some vocals on another track. Nas blew Jay-Z off, and then there was an issue with payments for the sample that Jay-Z used. As is often the case, it escalated with each of them taking shots at the other in their respective songs.
They finally mended fences in 2006 or so when Nas signed on with Def-Jam Records. Jay-Z was the President at the time, so they patched things up and then went out on tour together.