Salesforce feature release lifecycle is
Pilot -> Beta --> GA
Pilot is the first Phase of Public testing.
Successful Pilots features will often transition to the BETA phase.
Generally Available (GA):
Once a feature has passed the Pilot and BETA testing phases it will be included in a Salesforce release(Summer, Winter and Spring).
Developer Preview or Sandbox Preview is available ahead of a major Salesforce release when all preview sandboxes are upgraded to the next major Salesforce release so that the new features can be tested thoroughly in a Sandbox org.
Check Release Notes for additional information on the features.
This Phase includes all the features for the Release too.