The Internet of Everything is primarily a marketing term used by Cisco, in contrast to the Internet of Things which is a more generic term used by many groups and manufacturers. Since IoE is used almost exclusively by Cisco, their definition should be quite reliable:
The Internet of Everything (IoE) brings together people, process, data, and things to make networked connections more relevant and valuable than ever before-turning information into actions that create new capabilities, richer experiences, and unprecedented economic opportunity for businesses, individuals, and countries.
In practical terms, this boils down to a few key points, which I'll outline below.
Machine to Machine (M2M) vs Machine to People (M2P)
The Internet of Things is primarily associated with connecting machines and devices together so they can share data, e.g. connecting your light bulb to your smartphone so you can control it.
The Internet of Everything is a more holistic term that considers every step of the process: human interaction with the computer; the machine to machine interaction; the output to the human and the process in general.
IoE can be used in industry to describe the changes that come about as a consequence of interconnected devices, and talk about the new processes that factories can use to be more efficient.
The Scale of the Internet of Everything
The IoE can also encompass systems bigger than you traditionally find in IoT networks. One example on the Cisco website is a train network, which is far bigger than you'd find in a domestic IoT network (the train could have a whole IoT network of its own, then be connected to a bigger network!).
Overall, there isn't a lot of difference - it's just a marketing buzzword used by Cisco, but it's otherwise very similar to the term Internet of Things.