I have a MacBook with a 200GB partition and 50GB free space. When I boot from a USB stick to install Ubuntu 13.04 the installer gives me the option of "installing alongside Mac OS X" or "something else".
I'm happy partitioning drives etc but I'm just curious about what the installer does to the file system in the first option.
The "Install alongside Mac OS X" option installs Ubuntu on the free (With free I mean 'not occupied by a partition') space on your hard drive.
You can create 'free' space using Disk Utility under Mac OS X.
Ubuntu will partition the free space to have a partition for Ubuntu, and a 'swap' partition that Ubuntu sometimes needs to work correctly. (The swap partition will probably never be more than 4GB)
The swap area is recommended to be double the size of RAM on your PC.If you want more you can increase your swap area. Swap is used only if the RAM is of less size.