OpenComputers is a mod that adds persistent, modular, and highly configurable computers, servers, robots, and drones to the game. All devices can be programmed using Lua 5.2, allowing for systems with varying complexity depending on the usage.
To learn about how to use the manual, check out the page about the manual (that green text is a link, you can click it).
As mentioned above, computers in OpenComputers feature persistence, which means that a running computer retains its state when the chunk it is in is unloaded. This means that if the player moves away from the computer, or logs off, the computer will remember its last known state and continue from that point on when the player goes near the computer. Persistence works for all devices except for tablets.
All devices are modular and can be assembled with a wide range of components, just like computers in real life. Players who enjoy tinkering will be able to optimize devices to their heart's content. If desired, devices can be dismantled and rebuilt if the initial configuration wasn't satisfactory. For computers and servers, components can be swapped out on-the-fly simply by opening the corresponding GUI.
OpenComputers devices are compatible with many different mods for manipulation of blocks and entities (through the adapter, or specific upgrades in a robot or drone). Power can be supplied using a large range of other mods, including, but not limited to, Redstone Flux, IndustrialCraft2 EU, Mekanism Joules, Applied Energistics 2 energy as well as Factorization Charge.
Devices in OpenComputers have extra features as well as some limitations. Computers are the base-line, and are able to take a fair number of components, controlled by the CPU tier being used. Computers also have access to components on all six sides. Servers are able to connect to more components (internally or externally) than a computer, through the use of component buses; however, due to the rack, the server is only able to access components from a single side of the rack, as configured in the rack GUI. Microcontrollers are further limited (compared to computers) by their lack of hard drive and disk drive slot, which means OpenOS can not be installed on a microcontroller. Microcontrollers do have a slot for an EEPROM, and can be programmed with a more focused operating system for a limited set of tasks.
Robots are moving computers, and are able to interact with the world (but cannot interact with external OpenComputers blocks). Unlike computers, once a robot is built, the components inside the robot cannot be removed. To circumvent this limitation, robots may be built with upgrade or card containers, allowing for on-the-fly swapping of cards or upgrades, if needed. OpenOS can be installed on robots by placing a disk drive in a container slot, which will allow insertion of floppy disks, or by placing a hard drive with OpenOS pre-installed in one of the hard drive slots. To fully reconfigure a robot, it will need to be disassembled first. Drones are limited versions of robots. They move differently, contain fewer inventory slots, and lack an operating system (similarly to microcontrollers, drones can be configured with a programmed EEPROM for a limited set of tasks). For the most part, robots and drones share the same upgrades and components; however, upgrades behave differently in drones, such as inventory upgrades only providing 4 slots per upgrade, for a total of 8 slots, while robots are capable of taking more inventory upgrades (total of 4) as well as getting more slots per upgrade (16 slots per upgrade).
This manual contains detailed information regarding all blocks and items, how to set up different types of systems and devices, as well as an introduction to Lua programming.