Salesforce feature release lifecycle is

Pilot  -> Beta --> GA

Pilot:

Pilot is the first Phase of Public testing.

BETA:

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:

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.

Note:

This Phase includes all the features for the Release too.