Solr is a search engine. Samvera creates an index document (basically a Hash) for each PCDM object in Fedora and puts it into Solr. This allows objects to be searched and discovered by a user. ActiveFedora also uses Solr to run queries such as “Get me a list of objects that are Images”
Fedora Commons Repository (a.k.a. Fedora, a.k.a. FCRepo)
Fedora is a data store that uses the LDP protocol to store linked data and binary files.
Rails is a web application Framework for Ruby.
Redis is a key-value store that we use to back our worker queues (using Resque or Sidekiq). Hyrax also uses Redis for user notifications.