For a cluster, there are a lot of metrics for Memory that I'm trying to understand, and i'm much confused with the workload and usage. What is the difference between them ?
Workload is a vR Ops synthetic value that is derived using multiple raw metrics that, when combined together, gives you a true indicator of how busy/utilized a resource is.
Workload, as a object-wide indicator, is configurable using profiles. These profiles allow you to decide 'which' raw metrics to consider as part of the "workload" status. Whereas more specific workload indicators, such as VM>CPU>Workload is going to be more reliant upon an underlying formula that is NOT user configurable and defined within the Management Pack itself.
Usage% is going to merely be a raw vCenter metric, calculated by vCenter Server and merely represented within vR Ops.