The partitions are quite clearly described on your screenshot.
sda1
is labelled "Recovery". Recovery partition allows to reinstall Windows and restore the computer to its factory default state. If you won't need Windows in the future, you can delete it.
sda2
is the EFI partition which is needed to boot the operating systems in modern computers that use UEFI and not the legacy BIOS to boot the operating system. Don't delete it or your computer will not boot.
sda3
is the Microsoft reserved partition which is used for additional system data that may be used by Windows. If you won't need Windows in the future, you can delete it.
sda4
is probably (as you suggest) the main Windows partition. To be sure, you can just mount in in Disks application and see what's on it.
I'm not sure what is the sda9
partition - from the screenshot it looks that it is located at the very end of the disk. But because it is NTFS formatted, and flagged as msftdata
(like sda4
) it is probably also something used by Windows, maybe another recovery partition? Probably you can delete it.
sda8
is a bit mysterious. Looks like something you created during Ubuntu installation (as it is ext4 formatted), but why isn't it mounted? Anyway, you said you know what this partition is, so no issue here.
sda6
is a root filesystem of your Ubuntu installation. Of course you should not delete it.
sda7
is a swap partition that your Ubuntu is currently using (it has a key icon next to its name, which indicates it is in use). Don't delete it as well. You can, however, assign swap space to a different partition or to a file on main Ubuntu partition, then it won't be needed anymore.