Some snippets of a slightly older (2012) but still accurate article at the Film Journal International:
First and foremost, the DCP [Digital Cinema Package] is a file format and collection of digital files for storing and conveying DC (Digital Cinema) audio, image and data streams for playback on theatre projector systems. DC, once the umbrella term for all films in digital (HDCAM, Blu-ray, DigiBeta, DVDs, files off Quicktime, etc.), now refers in the U.S. to DCPs in theatres. Because of this association, DC is also distinct from HDTV (High-Definition TV) because it is not dependent on television or HD (high-definition) video standards, aspect ratios or frame rates. It is also distinct from E-Cinema, a term largely used for theatres overseas in places like India and Japan where digital standards differ from those set by DCI, the digital-cinema initiative co-venture of the major Hollywood studios behind the DCP standards.
Created in 2002, DCI emerged on the heels of work done in 2000 by SMPTE (the Society of Motion Picture and Television Engineers) when it became clear that for big pictures on big screens, something more than HDTV was needed. DCI, working with SMPTE on the formatting of the data and with the American Society of Cinematographers, published its final specifications...
The completed DCP, which began either digitally or captured photochemically, typically arrives in theatre booths stored in a small, plastic-encased hard-drive (think flash drive, only bigger and more square), typically a five-pounds-shy Cru Dataport DX-115 caddy that also typically loads into the server (audio pioneer Dolby plays one of the major roles in the server business) for ingestion and interface with the DCP-compatible projector. Another projection booth co-star, the IMB (the Integrated Media Block), contains a DCP’s data files of audio, subtitle and encryption information and decrypts and decodes the DCP itself.
And no show goes on without the KDM (the key delivery message) that unlocks the DCP for showing and is specific to the booth’s server and projector and the theatre’s screen. The keys are digit codes that activate the content for a certain amount of time (maybe only a few hours or even open-ended for long runs) and are controlled and generated by a film’s distributor.
And, FYI, as long as we’re on an acronym tear, let’s not omit exhibition’s wake-up call to what was going on. It was in 2007 that AMC, Regal and Cinemark forged their DCIP (Digital Cinema Implementation Partners) co-venture to ease their transition from film to digital.
Mainly, it used to be hard drives physically delivered. Fiber Optic or Gigabit internet downloads are newer and common as well. Either way, it's encrypted, and requires the keys to decrypt on specially manufactured servers.
The encrypted files and keys are specific to the server/theater they are intended to be screened at, no two copies are alike.