When you right click on a folder or file > Select 'Properties' > Security tab:

Sometimes you see three or more group or usernames in the Listbox as shown above. What's the difference between them? How is the permissions for my own user account separated from "Administrators" group? And What's the SYSTEM is for?

Sometimes you see three or more group or usernames in the Listbox as shown above. What's the difference between them?

The list you are looking at is called Access Control list or ACL.

Each line in this list represents Access Control Entry or ACE. ACE specifies type of access. It can be: allow/deny, read,write,delete, inherit from parrent object and so on.

What's the difference between them?

Various 'security principals' may be given various level of access. Security principals are users, computers, groups.

How is the permissions for my own user account separated from "Administrators" group?

If your user account is in the Administrators group, then it is not separated. If it is not in the administrators group, then, of course it is separated from the administrators group. Users who are not in the Administrators group are more secure against attacks of malware. Although they are not in the aAdministrators group, they can still perform administrative tasks by using 'Run As administrator' feature and providing credentials for administrative account.

And What's the SYSTEM is for?

This is the most powerfull security principal in your computer. System processes run under this account. Check services.msc console.

How do I change the permissions for a file to give users the full control?



Open file or folder properties.

Click Security tab. The picture you posted will open.

Click Edit button.

Click Add.

In the picker window, type user or group to whom you wish to assign permissions. The user or group must already exist.

In the permissions, click Full Control.

Click Apply

Click OK to close all open windows.

 

Thanks for the helpful answer. I'm still confused about something.

As you said that I need to type that user or group i.e. the name of account which I want to assign permissions to. But there are already "Users" and "Administrator" groups. So, Why do I need to add my own and what are they for?

For example: Lets say, I want to add my username "Parcx" into the group. I may have account with standard or administrator privileges. So, why don't I already belong to  "Users" and "Administrator" group? In other words why do I need to add "Parcx" (my username) into the ACL?

I don't understand the function of "Users" and "Administrator"  groups which are already there.