Net Neutrality, as defined by the Open Computing Facility is "A level playing field for Internet transport. It refers to the absence of restrictions or priorities placed on the type of content carried over the Internet by the carriers and ISPs that run the major backbones. It states that all traffic be treated equally; that packets are delivered on a first-come, first-served basis regardless from where they originated or to where they are destined”
Net neutrality became an issue as major search engines such as Google and Yahoo! increasingly generated massive amounts of traffic compared with other sites. It also became an issue because some carriers that offered subscription-based VoIP services were also transporting their competitors' VoIP traffic.
The short video below summarises Net neutrality well