A Node is a fixed size chunk of secure network-attached RAM.

Nodes can be launched using different instance types. Changing instance type allows Vertical Scaling.

Due to the different nature of Memcached and Redis, Elasticache managed them quite differently.

A Shard (Redis) is a group of up to 6 ElastiCache nodes

A Cluster (Redis) is a group of 1 (Cluster Mode disabled) or up to 90 Redis Shards (when Cluster Mode is enabled) in a single or multiple Availability Zones.

Data is partitioned across all the shards in that Cluster.

with Cluster mode Enabled you can have multiple Shards (within same or across different Availability Zones) where you have Primary node and zero to 5 Replicas.

With Cluster Mode enabled you can also choose between online or offline resharding

online resharding you can add or remove shards you can scale horizontally by adding a new replica

A Cluster (MemeCacheD) are collection of one or more nodes. Amazon Elasticache automatically detects and replace failed nodes.

Scaling horizontally by adding nodes to a cluster.

For improved fault tolerance, locate your Memcached nodes across multiple Availability Zones (AZs) within the cluster's AWS Region (but remember that each node is a partition of data and there is no replica nor backup).