[Marco van Herwaarden]

1. 'chmod' is a *nix commando.
2. It changes permissions on files
3. You can do this from SSH/Telnet prompt and most FTP clients will also support it.

A small explanation on these numbers:
There are 3 permissions you can set on each file/directory:
- Read permission
- Write permission
- Execute permission (or list contents if it is a directory)
These 3 permission are put together as a binary number:
- 1xx - For the read
- x1x - For the write
- xx1 - for execute
This binary number can be written as a decimal number again:
binary 111 = decimal 7 = Can read, Can Write, Can execute
Binary 100 = decimal 4 = Can read, Can not write, Can not execute

Now we have 3 levels of users we can control. First of all each file is owned by a user and a group. Now we can control the following levels:
- Owner - The user that owns the file
- Group - Any user that is member of the group that owns the file
- Others - Anybody not one of the above.

Now we can write the decimal numbers we calculated above behind each other for each level of access:
750 - Means permission "7" for the owner, permission "5" for the group, permission "0" for others
Resulting in:
owner - Can read, can write, can execute
group - Can read, can NOT write, can execute
others - Can NOT read, can NOT write, can NOT execute

This is just a simplifie explanation, hope it helps understanding a bit more.