Switching To Ubuntu From Mac OS X
OS X is an Apple interface wrapped around a Unix core, whereas Ubuntu is a Unix interface wrapped around a Unix core. This makes switching from OS X much easier than switching from Windows, but still quite challenging. For example, the OS X terminal is almost as powerful as the Linux terminal, but you'll need to use far more of that power in Linux.
The most obvious difference between OS X and Ubuntu is when installing software, which is similar to the App Store in OS X Lion and Snow Leopard. In OS X, you usually buy or download programs in .dmg files that may contain .pkg software packages. The Ubuntu equivalent of a .pkg file is a .deb file, although you'll rarely see one in practice.
Ubuntu has a built-in package management system, and it's recommended to install programs from there. See the install software guide for more information.
Programs installed through Ubuntu are guaranteed to work with Ubuntu, and automatically updated when you upgrade to the newest version of Ubuntu.
In the same way that OS X only runs software designed for OS X, applications must be made for Linux to be able to run on Ubuntu. Most Linux software is available for free over the Internet. The following pages feature a small selection of popular applications available for free in Ubuntu: