I have been reading and I want to install Ubuntu. I saw it says that it can use all the hard drive and override the other OS. So my question is, will it delete everything in my hard drive and install itself or will it delete my OS only?
Yes, if -you- tell the installer to overwrite an OS it will format anything related to that OS. So, assuming Windows, C:, D:, E: and any other drive letters.
If the other OS is Ubuntu it will delete all partitions related to the OS.
If you have create "unallocated space" from the OS you use now you can use that to "install alongside" and you will have the new OS together with another OS. That will not delete your old OS.
As with any action that involves changing the disk layout: make a backup of your important files on any of the operating systems. It is even better to have a regular backup schedule.