Even point-and-shoot cameras usually include a tripod socket. A system camera includes at the very least a camera body and separate, interchangeable lenses, whence the alternative name interchangeable-lens camera (ILC). In addition it often includes:
Adapters for third-party or legacy lenses, including tilt-shift adapters
For film cameras, a motor drive to advance the film automatically.
For film cameras, different camera backs, e.g. large capacity for bulk film, or data back or datebacks for recording exposure and date information.
Add-on correction or magnifying lenses to help focusing or composition through the viewfinder.
Interchangeable viewfinders, including interchangeable focusing screens in the case of SLRs. Viewfinders and focusing screens could give different metering options.
Extra-capacity battery packs, often in the form of a "battery grip" (a second, portrait-oriented handgrip), but also including fully external packs with cable interface
AC adapters with dummy-battery interfaces for studio or other stationary work
Sockets and matching cables for direct output, e.g., to a television
While some early mechanical interfaces are standardized across brands, optical and electronic interfaces are often proprietary. Hot shoes have a common interface for basic flash functions, but often contain proprietary contacts inside for advanced flashes and data modules.