Ethereum Milestones

Several prototypes of the Ethereum platform were developed by the Foundation, as part of their Proof-of-Concept series, prior to the official launch of the Frontier network.

The last of these prototypes culminated in a public beta pre-release known as "Olympic".[17][18] The Olympic network provided users with a bug bounty of 25,000 ether for stress testing the limits of the Ethereum blockchain.
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSUyPwzespbamzbI2DpU74bWFO3VqplX4VSfSzya81qJk57aoq-vw
After Olympic, the Foundation announced the beginning of the Frontier network to mark the tentative experimental release of the Ethereum platform in July 2015. Since the initial launch,

Ethereum has undergone several planned protocol upgrades called milestones, which are important changes affecting the underlying functionality and/or incentive structures of the platform.

The current milestone is named "Homestead" and is considered stable includes improvements to transaction processing, gas pricing, and security.

There are at least two other protocol upgrades planned in the future, i.e. Metropolis and Serenity. Metropolis is intended to reduce the complexity of the EVM and provide more flexibility for smart contract developers.

The move to Serenity is still uncertain, but should include a fundamental change to Ethereum's consensus algorithm to enable a basic transition from hardware mining (proof-of-work) to virtual mining (proof-of-stake) Improvements to scalability, specifically sharding, are also said to be a key objective on the development roadmap.