Chmod / File Permissions Calculator

It gives the owner full read/write/execute access while group and other members get read and execute only, no write. That's the standard setting for directories and executable scripts other users need to run but shouldn't be able to modify.

644 has no execute bit for anyone, correct for regular files like images or text documents. 755 includes execute, needed for directories (to enter them) and scripts/programs (to run them). Setting execute on a file that doesn't need it is a common, mostly-harmless but unnecessary permission mistake.

Almost never on anything reachable from the internet. It gives every user on the system full read, write, and execute access, which is a real security risk on a shared or public-facing server. It's sometimes used as a quick fix for a permissions error, but the actual fix is almost always setting the correct, narrower permission instead.