HCI are specific solutions that combine computing and storage (and sometimes also networking) capabilities from more hosts, to have a shared pool of resources. A vSphere cluster already does this for the computing part. Some storage products extend this to the storage part, making more external storage unnecessary, and making the HCI market a relevant trend; it is not only growing fast, with more attention from the big storage vendors, but it is also changing fast.

Some HCI products compatible with vSphere 6.5:

• Nutanix: This is the pioneer of HCI, actually it is the only solution that can work with all the hypervisor technologies (VMware vSphere, Microsoft Hyper-V, KVM with the custom AHV, Citrix Xen Server).
• HPE SimpliVity: In January 2017, HPE acquired Simplivity to bring HCI into its portfolio. Available since April, the HPE SimpliVity 380 (based on HPE ProLiant DL380) is the first HCI product after this acquisition. It offers a full suite of traditional IT functions including WAN optimization, unified global VM-centric management, data protection, cloud integration, built-in backup, disaster recovery, caching, and scale-out capabilities. In-line deduplication, compression, and optimization are applied to all data at inception.
• NetApp HCI: The first HCI from NetApp, based heavily on NetApp's SolidFire all-flash storage technology (NetApp acquired SolidFire in late 2015).
• Pivot3 Acuity: This is claimed to be one of the industry's first priority-aware and policy-based HCI, offering NVMe PCIe flash performance, giving customers the ability to consolidate multiple mixed-application workloads onto a single infrastructure.
• VMware vSAN: The only in-kernel HCI solutions for VMware vSphere, totally integrated into vCenter management; with more than 10,000 customers in all verticals and growing.

Note that there are also some specific HCI solutions for ROBO scenarios, or small clusters (usually HCI starts from at least three nodes); for example, from StarWind or StorMagic.