Dropbox allows you to keep a constantly updated copy of a folder of files on your computer (and any sub-folders) on a website online. You can also use it for sharing files with other people. If you are using Dropbox to share files with others, keep in mind that deleting the file from your computer or moving it out of the Dropbox folder will also make it unavailable to friends who have not yet downloaded it.
At the free level, you get 2 GB of space, but you get free extra space when you refer other users (up to a maximum of 16 GB). You can also buy extra space.
You can sign up for dropbox at http://db.tt/tAAx5Xsg (That is a personal referral that will give you 250 MB of bonus space.) You will need you install a program on your computer, which will make a folder called Dropbox that is constantly synchronized with your Dropbox account online.
If you put files in the Public folder of your Dropbox folder on your computer, they will automatically upload to your Dropbox account and you can get a public link. Right-click on the file that you want to share, and under "Dropbox" find "Copy public link". Like this:
After that, you can go to any forum or your email and paste the link.
You can also browse your files on the website and copy the link to your clipboard there. It has to be in your Public folder, or in a subfolder inside your Public folder:
This will also give you the option of shortening the link, although that kind of obscures what it goes to. It will look similar to this:
Remember that your Dropbox always contains a copy of the Dropbox folder on your computer. If you delete the file on your computer, it will delete from your Dropbox and the link will no longer work.