While researching Redis Cluster I found a large number of tutorials on the subject that confused Replication and Cluster, with people setting up 'replication' using cluster but no slaves, or building a 'cluster' only consisting of master-slave databases with no cluster config.
So to clear things up:
Replication involves a master server which serves reads and writes, and duplicates all data to one or more slave servers (which serves reads but not writes). Slaves can be used to replace a master in case of failure, spread read request load, or to perform backups of the database to reduce load on the master.
Clusters are used when you have more data than RAM in a single machine: the data is automatically split (based on the key) across multiple databases, increasing the amount of data you can store. Clients requesting a key from any cluster node will be redirected to the node holding the key, and are expected to learn the locations of keys to reduce the number of redirects.
Replicaton + Cluster
Redis Cluster supports replication by adding slaves to existing nodes, if a master becomes unreachable then its slave will be promoted to master.
Last but not least, Redis Sentinel can be used to manage replicated servers (not clustered, see below.) Clients connect to a Sentinel and request a master or slave to communicate with, the sentinels handle health checks of the masters/slaves, and will automatically promote a slave if a master is unreachable. You need to have at least 3 sentinels running so that they can agree on reachability of nodes, and to ensure the sentinels aren't a single point of failure.
Cluster handles its own promotion and does not need Sentinel in front of it.