Executable and Linkable Format (ELF) defines the structure for binaries, libraries, and core files. The formal specification allows the operating system to interpreter its underlying machine instructions correctly. ELF files are typically the output of a compiler or linker and are a binary format. With the right tools, such file can be analyzed and better understood.
Understanding ELF
https://linux-audit.com/elf-binaries-on-linux-understanding-and-analysis/