When i'm done using Windows/Ubuntu, can I just reboot and select the other from the boot menu?
Yes, but it is recommended that you have Windows check the disk before installing and correct any file system errors.
After installation you will be presented with a boot menu(supplied by Windows) to boot to either Windows or Ubuntu. You make your selection there and then it boots into the selected OS. If booting into Ubuntu you may also see the Ubuntu Grub screen. If so you will need to select the Linux version you want to run....the top is the newest under that will be "Previous Versions" that will allow you to boot into the older kernels if needed(after a fresh install there will only be the one selection, the Previous menu selection will appear if and when you update to the newer kernel.
The Grub can be shown or hidden by editing your /ect/default/grub file.....there are plenty of questions on askubuntu about doing that so I will not take the space here to repeat the information.
HERE is the instruction page for Wubi from the developers of Wubi it has rather detailed instructions and some FAQ's
HERE is the detailed Ubuntu page on Wubi that includes the same instructions as the above link and some advanced troubleshooting and a more detailed FAQ.
Yes, you can simply reboot to the other OS. It also uses Windows Boot Manager instead of GRUB.