What are file permissions in Linux?

Linux uses a Unix permission model with three permission classes: owner (user), group, and others. Each class has three bits: read (r=4), write (w=2), and execute (x=1). In ls -l output, permissions appear as a 10-character string like -rwxr-xr--: first char is file type (- file, d directory, l symlink), then 3 chars each for owner, group, others. Octal notation: 755 = owner rwx, group r-x, others r-x. Execute permission on a directory means the ability to enter it.