what the different between the auto mounter and /etc/fstab

The auto mounter will dynamically assign device files to new hardware as they are connected. Auto mounting systems may not always obey /etc/fstab's mount points or device files. Manually mounting devices will, unless overwritten in the mount command, follow the rules in /etc/fstab.

No, it won't. Udev is responsible for creating device nodes for newly connected hardware. Any auto-mounting software doesn't get into the picture until after device nodes have already been assigned.

For the record, there's no such thing as the auto mounter. There are lots of ways to automatically mount filesystems. In fact, I've personally used at least four different ways. Some are related to fstab, some aren't. If you want any meaningful comparison, you're going to have to at least specify which auto-mounting system you're talking about.

Thank you for correcting me. To the OP: please ignore my flawed explanation.

how to use auto monuter beside the /etc/fstab

That doesn't make things any clearer. For starters, why not tell us what you want to mount automatically (USB drives? CD drives? Network shares?) and the name and version of your distribution.

i want to know what is the auto mount and how can i use it and
is it alternatvie to /etc/fstab ?

thank you in advance

It's not an alternative--it will still be used by the system during boot-up to mount your local (and root) filesystems.

If you use it, what will happen is that you will plug in your block device (i.e. a hardrive, a usb stick, a card reader, etc) and Linux will try to mount it for you, usually in /media or /mnt.

What Linux distro are you using? We may be able to help you find an automounter package, if you choose to use one.