Open System Preferences, click on the Accounts icon, then click on the LogIn Items tab. Log In Items are set in the Accounts preferences. Startupitems are usually installed in the /Library/StartupItems/ folder and less often in the /Home/Library/StartupItems/ folder.
Some applications may install a startupitem or a Log In item. Again, they don't do anything but take up disk space once the application is trashed. You can also delete the folder that's in the Applications Support folder. You can also check there to see if the application has created a folder.
Some applications may install components in the /Home/Library/Applications Support/ folder. In some cases the uninstaller may be part of the application's installer, and is invoked by clicking on a Customize button that will appear during the install process. Some applications may install an uninstaller program that can be used to remove the application. If you want you can look for them in the above location and delete them, too. Although they do nothing once you delete the associated application, they do take up some disk space. Applications may create preference files that are stored in the /Home/Library/Preferences/ folder. Most OS X applications are completely self-contained 'packages' that can be uninstalled by simply dragging the application to the Trash.
