What is the difference between maintenance and standby in pacemaker

Red Hat, being a major contributor of pacemaker (SuSe being an other, from Clusterlabs' informations), provides quite good documentation about it.

• standby mode is for a node being a part of a cluster. Setting a node in standby mode will (attempt to) migrate services on other online nodes.

  4.4.5. Standby Mode
  The following command puts the specified node into standby mode. The specified node is no longer able to host resources. Any resources currently active on the node will be moved to another node. If you specify the --all, this command puts all nodes into standby mode.

  To set a node in standby mode:

  pcs cluster standby <node>
  

• maintenance-mode is for the whole cluster. Setting the cluster in maintenance mode will make the cluster not attempt to manage services anymore for whatever reason: the administrator of the cluster can now do actions without having the cluster interfering in any way.

  4.8. Cluster Maintenance
  In order to perform maintenance on the nodes of your cluster, you may need to stop or move the resources and services running on that cluster. Or you may need to stop the cluster software while leaving the services untouched. Pacemaker provides a variety of methods for performing system maintenance.
  [...]
  If you need to put the cluster in a state where no services will be started or stopped, you can set the maintenance-mode cluster property. Putting the cluster into maintenance mode automatically unmanages all resources. For information on setting cluster properties, see Table 12.1, “Cluster Properties”.

  To set the cluster in maintenance mode:

  pcs property set maintenance-mode=true
  

So for example, to compare two things which might appear similar: the main difference immediately visible between putting all nodes in standby mode, and putting the cluster in maintenance mode, is that for the former case, it will stop all services, since there's no online node anymore, while for the later, services will stay as they were, started or not, on various nodes.