What is SYNC?

   File synchronization (or syncing) in computing is the process of ensuring that computer files in two or more locations are updated via certain rules.

 
    In one-way file synchronization, also called mirroring, updated files are copied from a 'source' location to one or more 'target' locations, but no files are copied back to the source location.
    In two-way file synchronization, updated files are copied in both directions, usually with the purpose of keeping the two locations identical to each other.File synchronization is commonly used for home backups on external hard drives or updating for transport on USB flash drives.
    The automatic process prevents copying already identical files and thus can be faster and save much time versus a manual copy, and is less error prone. However this suffers from the limit that the synchronized files must physically fit in the portable storage device. Synchronization software that only keeps a list of files and the changed files eliminates this problem.
Here are some websites allow you to syncing

• Windows Live Mesh 

  With Windows Live Mesh and the Devices website, you can keep up-to-date copies of documents, photos, and other files on all of your computers, whether PC or Mac. Sync your folders to the 5 GB of free storage space you get with SkyDrive, and work your files on the web from any computer. Run programs and browse all the files on your PC from anywhere by connecting remotely. And keep your favorites in Internet Explorer and your Microsoft Office settings up to date on all of your PCs by syncing your program settings.

  Sync folders between computers

  If you have two or more computers with Windows Live Mesh installed, you can sync folders between them (peer-to-peer). When you sync folders between computers, any changes you make on one computer will be made on the other whenever the computers are online at the same time. The contents of the synced folders are saved on all of the computers, so you can still work on them even when you're not connected to the Internet.
  All transfers are encrypted using the Transport Layer Security (TLS) or Secure Sockets Layer (SSL) protocols.
  Click the Start button. In the search box, type Mesh, and then, in the list of results, click Windows Live Mesh. Select the folder you want to sync, and then click Sync.
  Select the other computers where you want to sync the folder, and then click OK. The folder will be saved in your personal folder on any PCs you selected. (You can get to this folder by clicking your name on the Start menu.) The folder will be saved in your home folder on any Mac you selected.
  
  
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• Apple iCloud

Price: Free to $100 per year for 100GB; additional $24.99 per year for iTunes Match
iTunes Match basically scans your music and, if it has a match of a song, it gives you license to download or stream that song from its servers at any time, meaning you can delete files from your computer.

•  CX

    Price: Free (10GB) to $19.99 per month (100GB)
It doesn't have a big name, or even a very long one, but CX offers more free storage than any of its competitors: a whopping 10GB! That will be reason enough for many users to snap up an account. CX also has a very elegant online dashboard that I like even more than its downloaded software for both Mac and Windows, which might make it ideal for people who used shared computers frequentlyand can't always install software locally.   

•   Dropbox 

Price: Free (2GB) to $19.99 per month (100GB)
Dropbox is ideal for working with files that you want store in a single folder. The Dropbox mantra is, "Put it in the Dropbox folder, and it will sync." However, Dropbox does allow you to set up entire sub-folder structures, if you'd rather, it's just not how the developers envisioned the service in its earliest days and hence is seen as an advanced option. When used as intended and designed, Dropbox is dead simple. There are versions for Windows, Mac, Linux, iOS, Android, and BlackBerry.

• SugarSync

 Price: Free (5GB) to $14.99 per month (100GB)
File-synchronization service SugarSync offers a generous 5GB cloud storage space with a free account. By far the most intuitive of all the products we've tried, SugarSync is a breeze to use across all major operating systems: Windows, Mac, Linux (in beta), Android, BlackBerry, iOS, Symbian, and Windows Phone. When you install the software on a computer, it adds a right-click function that allows you to quickly mark any file or folder to back up, which is immensely useful.

• Syncplicity

 
 Price: Free for 2GB to $15 per month for 50GB
Similar to SafeSync, Syncplicity also touts its strong security measures and puts a price on its syncing service ($15 per month for 50GB), although it does have a free option that's equal to Dropbox's: 2GB. If you use sites such as Picnik, Zoho, Scribd, and iPaper for editing files, Syncplicity might be appealing for its integration with those services.