I am sure you have asked this question to yourself many times. Well, actually, you probably never have. The answer is that everyone should backup and, if you use more than one device, you need to sync, too. ChronoSync can synchronize and backup. This tip explains the difference between the two and gets you started using ChronoSync.
First things first:
Locations
The location where all the data is stored on a Mac is the hard drive. Hard drives are also referred to as Disks, Volumes, and HD's. Data can also be stored on external hard drives, iPods, iPhones, mobile phones, web servers, discs, and a lot more.
Files
All of the data on your hard drive, or any device for that matter, is really just a bunch of files. These files have many descriptive names: Applications, Packages, Utilities, Folders, Documents, Invisible, and many more. For the most part your Mac can be divided into four basic sections; Applications, Library, System, and Users. Each of these sections is a folder at the root of your hard drive that stores a bunch of files.
The difference between Syncing and Backing up:
When you need to use files on both locations - the source and the destination - you should synchronize them. Files get copied from a source to a destination and then back again.
You backup files when you need to use files in one location but want a backup of the files in a second location in case something goes wrong. When backing up, files get copied from a source to a destination. You typically never use the files on the destination other than to restore them to the source.
So what have learned? Your Mac has a hard drive. On the hard drive you have a bunch of files. All the users on the Mac have a Home folder with all their files in it. When you synchronize or backup, you are simply copying files from one location to another. The two sections below point you in the right direction for setting up ChronoSync to synchronize and/or backup.
Sync It!
You should synchronize any data you need to access on other devices like other Macs, iPhones, iPods, or even PCs.
You don't want to synchronize System or Library files outside of your Home folder because these files can vary from Mac to Mac. You don't want to synchronize Applications because all Applications should be installed and updated from an installer or Software update. You should only synchronize files in your Home folder and, more importantly, you should only synchronize some of the files in your Home folder using ChronoSync. There is some Managed Data inside your Home folder you should not sync using ChronoSync. Managed Data are files in your Home folder that are structured into databases like your Address Book, iTunes, iPhoto and Bookmarks. To learn the basics of synchronizing, read the tip Quick Setup and to learn how to sync your Home folder, read the tip Syncing your Home Folder.

If you want to find out more information on synchronizing your Managed Data then get the book
Take Control of Syncing Data in Snow Leopard by Michael Cohen.
NOTE: In some circumstances it may be fine to synchronize files outside of your Home folder. This usually only the case if you are dealing with servers or unique situations. This should only be done by the advanced Mac user. So if you do need to synchronize files outside of your Home folder contact us to go over your scenario.
Back it Up!
Unlike synchronizing, which limits what files you can sync, you can (and should!) back up all the files on your Mac. If you fail to regularly do this, eventually you may be one of those people we all hear about who lost their data because they were not diligent about backing up. You can backup to internal drives, external drives, Macs, PC's, iPods, iPhones, or web storage. Most users use Time Machine to backup their Mac and if you do that you are all set. However, Time Machine does have some limitations that ChronoSync does not have. For one, it can only back up your Mac. It cannot back up data stored in other locations. Second, and most notably, it cannot make a bootable backup.
On the other hand ChronoSync does have one limitation, too. Although ChronoSync archives deleted and changed files, it does not have the ability to restore these files by a specific time like Time Machine can. Instead it allows you to restore files by file version. So every time a file gets deleted or changes, ChronoSync archives the older version in case you need it again. To learn the basics of backing up read the tip Quick Setup and to learn all about bootables read the tip Creating a Bootable Backup.
If you want to find out more information on backing up your Mac then get the book
Take Control of Easy Mac Backups by Joe Kissell.