If you've come to Ubuntu from Mac OS X, welcome, and thanks for installing Ubuntu! OS X and Ubuntu actually have a lot in common, and they're both designed to "just work." This will probably make it easier for you to transition.
This document intends to give you some information, resources and terminology to help you discern the similarities and differences between OS X and Ubuntu.
Just like OS X, Ubuntu comes with a useful selection of applications by default, such as an office suite, web browser, multimedia applications and games. Both use software packages to install software. With Ubuntu, however, you usually need to install packages, as opposed to downloading the packages and using them without installation. When installing packages in Ubuntu, you will need to supply your password, just like when installing updates or certain programs in OS X.
Much of the software that you'll find yourself using is contained in the Ubuntu package repositories. That means no longer needing to go online to sites like VersionTracker or MacUpdate, unless you are looking for something uncommon. If you are looking for something less common, then Sourceforge is a good website of open source software projects.
See InstallingSoftware for more information on how to install software from the repositories and from online sources.
In OS X, the main types of compressed archives for files and folders are .dmg (Disk Image) and .sit/.sitx (Stuffit). Ubuntu can't natively read .dmg archives, but the free dmg2iso utility can convert .dmg archives to .iso. Ubuntu can read .sit, .sitx, .zip, .tar, .gz, .tar.gz, .bzip and a number of other archive formats.
Mark Pilgrim's list of Ubuntu essentials for those switching from Mac OS X. Includes Linux equivalents of popular Mac programs and Mark's opinions on different Linux apps. Essentials, 2006 Edition