These docs cover everything from setting up and running an etcd cluster to using etcd in your applications. Improvements to these docs are encouraged via etcd on GitHub. For more in-depth support, jump into #coreos on IRC, email the dev list, or file a bug.
Reading and writing into the etcd keyspace is done via a simple, RESTful HTTP API, or using language-specific libraries that wrap the HTTP API with higher level primitives. You may find the API reference and the interacting with etcd guide useful starting points.
Configuration values are distributed within the cluster for your applications to read. Values can be changed programatically and smart applications can reconfigure automatically. You'll never again have to run a configuration management tool on every machine in order to change a single config value.
High level design goals and in-progress features are tracked on GitHub.
Benchmark docs for etcd.