Once you hit level 70, the source of end-game gear will be Command Crates from Galactic Command.
Most activities in the game will earn Command Experience Points (CXP), which will earn you Command Ranks. Each time your Command Rank increases, you earn a Command Crate.
The higher your Command Rank, the better gear that will drop from your Command Crates.
The highest difficulty Operations and Uprisings, along with Ranked Warzones are intended to be the fastest ways to earn CXP. This means they are the fastest way to get the best gear.
Both PvP and PvE gear will come from Command Crates. Their gear is now shared as Expertise is being removed (head to this thread to discuss PvP/PvE itemization specifically).
Gear will no longer drop from bosses as all gear will come from Command Crates. All cosmetic/unique drops will still remain on those bosses (Stronghold Decorations, Wings of the Architect, etc.).
Players will be able to craft comparable item level gear without set bonuses.