Basically you have four kinds of things that can be on your laptop in terms of software:
Applications -> These are the programs you are familiar with like Safari, Mail, iPhoto, Photoshop, Word, Excel, Powerpoint, or iWork.
Documents -> These are the files you create with your favorite applications or manage with them.
Preferences -> These are created by applications to store things like memory of your registration keys, bookmarks in browsers, etc...
System files -> these are the programs that Apple installs or other people install to run in the background with their applications for all people using your machine.
When you are talking about cleaning your laptop of unnecessary things, you have to decide, is there a chance that what you are going to clean, you may need at a later time? So before cleaning, I always recommend backing up your data to a second external source such as an external hard drive the capacity of your internal hard drive. A cloning software such as Carbon Cloner makes a copy of all four essential pieces of software that are sitting on your machine at the time the clone is made. These take a couple hours to make and are typically best done at night. That way if indeed you delete an essential piece of software, you can always recover it again from your backup. The other thing to remember, is backups are necessary in anycase prior to any upgrading or updating of your system. Software can get corrupted, the internal hard drive can eventually fail. Having a backup plan is essential.
Once you are backed up at least once, and preferably to two external distinct sources, then you can go about deleting non-essential applications from the Hard drive -> Applications folder. If there is a chance you might have to reinstall the said application at a later time, it is better to run an uninstaller program made by the application vendor. Alas, the Application is not always just in the Applications folder. Apple has three Library folders, and a Documents folder that may keep track of other files that certain applications store. Before removing any software you may need to reinstall double check with the Application vendor where they store all their documents so you can remove all traces. Otherwise when you go to reinstall the application, it may have some incompatible preference left over.
Another approach to deleting unwanted data is looking at your iTunes and iPhoto for duplicate music and photos. If you created iMovies the same thing you might want to do is empty your Movies folder of movies you no longer want to keep.
So backup first, then determine what you want to remove.
