A Hopfield network is a form of recurrent

Recurrent neural network

A recurrent neural network is a class of neural network where connections between units form a directed cycle. This creates an internal state of the network which allows it to exhibit dynamic temporal behavior. Unlike feedforward neural networks, RNNs can use their internal memory to process...

 artificial neural network

Artificial neural network

An artificial neural network , usually called neural network , is a mathematical model or computational model that is inspired by the structure and/or functional aspects of biological neural networks. A neural network consists of an interconnected group of artificial neurons, and it processes...

 invented by John Hopfield. Hopfield nets serve as content-addressable memory

Content-addressable memory

Content-addressable memory is a special type of computer memory used in certain very high speed searching applications. It is also known as associative memory, associative storage, or associative array, although the last term is more often used for a programming data structure...

 systems with binary

Binary numeral system

The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...

 threshold units. They are guaranteed to converge to a local minimum, but convergence to one of the stored patterns is not guaranteed. Furthermore, Hopfield networks provide a model for understanding human memory.
The units in Hopfield nets are binary threshold units, i.e.