Ubuntu by default uses two partitions, one of type
ext4 and the other of type
Swap. Windows uses NTFS partitions not useful for Ubuntu installation.
If you have a free partition in Windows, delete it using Windows Disk Management Tool. Keep the free space unallocated.
When you install Ubuntu, choose the Install side by side Windows option. Ubuntu will find the unallocated space and create the partitions it needs.
See Ubuntu installation on Windows7 with D partition for more details.
The default installation of Ubuntu includes a boot-loader called GRUB. Every time you boot GRUB will load before Windows or Ubuntu and give you the choice to boot either Ubuntu or Windows. If you do not choose anything, it will boot Ubuntu after waiting for a while. Both the time it waits and the default boot can be changed later.
Hope this helps