Infiniband is a type of computer network which has extremely high throughput and low latency, which makes it a great solution for the construction of computing clusters. Infiniband has the ability of RDMA, which is Remote Direct Memory Access - access to the part of the other server's memory without need to use drivers and operating system (as much as it's possible). This mechanism is often used not only to communicate between computing processes, but also to transfer data, eg. on temporary disk resources or for checkpointing.
Nowadays, the most common type of Infiniband (also used in Cyfronet) is Infiniband 4X QDR, which offers troughput at the level of 40Gb/s and real latency in order of a few microseconds.