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wysiwyg
The wysiwyg (what you see is what you get) module allows you to plug feature rich editors into drupal's content editing process. This makes editing content similar to using a program like Word or OpenOffice. Read the docs to confirm, but installation should be as easy as extracting the archive into sites/all/modules, then enabling the module. You will need to download an editor and place it in sites/all/libraries/<editor name>.
Many editors are available to use. The TinyMCE editor works well.