If you install Ubuntu, you can install it alongside Windows. That means, whenever your computer starts, you will be greeted by a selection that asks you whether you would like to start Windows or Ubuntu.
You can also, however, remove Windows upon installing Ubuntu.
In the first case, all of your programs are kept exclusively on Windows. There is no synergy between the Ubuntu and Windows installation. That means no files or programs are shared between the two operating system. You can access files that are on the Windows system from Ubuntu per default. Windows requires you to install a driver (ext42fd) to be able to access Ubuntu's files.
In the second case, all programs and files are lost. You will need to back up your files before making the switch. Programs can never be transferred. So if you have Word on Windows, it cannot be moved to Ubuntu. First, because Word doesn't exist for Ubuntu (we have LibreOffice Writer). Second, because if a Word for Ubuntu existed, it would be incredibly different from the version for Windows under the hood.
So, if you install Ubuntu, you will have to reinstall programs that you currently have on Windows. If the program doesn't exist for Ubuntu, you will have to find an alternative program that functions approximately the same. So instead of Word, you would have Writer. Software like Skype, however, has both a Windows and a Linux version. This differs per piece of software.
If you decide to switch back to Windows after a while, and you had taken the approach of removing Windows, you will have to reinstall Windows entirely and reinstall all programs you had previously had. So if you have Word now, wipe Windows, install Ubuntu and reinstall Windows, you will have to install Word again.