Veeam Essentials is a bundle of Veeam Backup & Replication v6 and Veeam ONE (monitoring & reporting). It is a very cost effective suite for smaller organizations. Cheaper then buying the individual components. In the past Veeam Esssentials could only be used if VMware vSphere Essentials or vSphere Essentials Plus was used on the managed hosts.
Recently Veeam changed this policy. Veeam Essentials can be used to protect and manage (monitor & report) any vSphere edition. It can be purchased in 2-socket bundles (used to be 6-socket bundle). The maximum number of sockets is still limited to six. Also Hyper-V costs can be protected since the release of version 6 of Veeam B&R.
More info at Veeam.com