Is it the same thing as creating an hard disk image?
No, it's not. System Restore backups only few system files and other configurations (drivers, etc.). For sure it's better than nothing but it's not a backup feature.
If not,what software will I use to do?
Acronis True Image, or Norton Ghost.
This Hard Disk image looks like an cd-rom image file(.iso or similar)?
More or less... a big file that could only be opened by the program that generated it.
So will I have a big file,about 4 or 5 GB?
It could be compressed, but depends on how many files are you really backing up.
Can I store on multiple dvds if it's larger than 4.7 GB?
Sure, they allow splitting the backup big file.