1. The user id and group id of the process. These are used to determine if the process has permission to take certain actions.

2. Processes can change their user id and group id in order to do something as a different user or group. This relates to that. In your programs, there will be no differences between the real and effective ids.

3. Every file is owned by a user and a group. These are used to determine how a process can access the file.

4. A file is anything that can be in a directory. A plain file is one that's not a directory, sym link, socket, pipe or device (etc?).

5. A file is a directory if it was created using mkdir(2).

5. Named pipes, sockets, and devices are special types of files. See chmod(1) for info on the three special bits.

6. They return the difference between the time the script started and the time the file was modified, accessed or had its inode changed, as a fractional number of days.