You can download the latest version of the Dropbox client here: https://www.dropbox.com/install
Dropbox is available for PC, MAC, and Linux.
Adding & Syncing Files
Once you have installed the Dropbox client, you will see a new Dropbox folder on your computer. Add files to that folder to sync them to your Dropbox. Once a file is synced, you can access it from any computer, smartphone, or tablet.
Move, Don't Copy
It is best practice to move your files to Dropbox instead of copying them. It is best to avoid having multiple copies of your files to avoid confusion. Everything in your Dropbox folder is still on your local computer and availble for use, even if you are not conencted to the Internet.
While Dropbox is accessible through mobile web browsers, it is preferable to install a native app. Dropbox apps are available on iPhone, iPad, Android, & Blackberry. The mobile apps do not have built-in file viewers but instead depend on the native viewing capabilities of your device.
Your files are very safe with Dropbox. Syncing your files between your computers ensure that you not only have a copy on each computer but also on Dropbox's storage servers in the cloud. Dropbox uses the same security methods as banks, such as SSL and AES-256 bit encryption.
For further help, please refer to Dropbox's Help Center: https://www.dropbox.com/help/category/Basics