Printing in 3d requires the use of some form or shape of G-code, which is the programming language that most forms of digital fabrication use. G-code itself makes use of lines of various strings, which include letters and numbers alike. Each letter and number give different instructions to the printer, CNC router or whatever machine accepts it. For example the letters A, B and C indicate to the machine a change in position or incremental movement for the X, Y and Z axis respectively. Key codes are also part of G-code; G01 (with zeroes) tells the machine to move in a line, whereas G02 tells the machine to move in a circular motion.
Short for sterolithography file, this format was native to 3D Systems's printers. It has since then, become a very popular format to CAD model with. It can be opened with text editors, as the strings in each line dictate the formation of the model. When put through a slicer, .STL files, like other input formats, end up becoming G-code for the 3D printer.
Part of a package made for Terminator 2 and Jurassic Park, the obj format was made to simplify 3D modeling. This format allows for commenting in the files, and lets users figure out the scale from there; objs do not have unit information so they must be rescaled whenever they are placed in a slicer.
Commonly found after 3D scanning an object or person, PLY files are complex. They can hold information for color, transparency, texture, direction and more. Programs such as Meshlab can be used to manipulate or repair PLY files.
Windows 10 and later natively support the .3mf file. This format was created by Microsoft and supports licensing and digital rights management. Many slicers do not yet support this file type but files created in this can be converted to .STL.