This release is part of a major overall upgrade of the IOTA protocol. Most notably are the changes for Kerl (https://github.com/iotaledger/kerl) and the accompanying transition phase.
About the Transition Phase
A detailed document about the transition phase will be ready within 20 - 30mins. Please wait before upgrading.
About the Snapshot
As this release is part of the snapshot, for GUI users which use the GUI as a full node, please make sure that you delete your database. You can find the database folder in the following directories on all respective OS'es. Make sure to delete the mainnetdb and mainnet.log folders: