There are two ways to share a file on dropbox.
Both are very cool.
If you share a folder, anything placed in that folder, apears in both of
your dropboxes.  The down side of this that the space is also a part of your
total dropbox space.
Not a problem if you are sharing small things, but the space adds up if you
are sharing gigabytes of data.

2.  You can upload a file to your dropbox.  When the file has finnished
uploading, which you can see from your system tray; then you right click the
file, go to the choice that says " share dropbox link l" and press enter.
You will get a dialog like the file attatch dialog, where you are directed
to your e-mail program.
You have edit boxes because you are sending the person an e-mail message
with the link to download the file included.
They open the message, click on the link, and follow the standard download
process from their web browzer.
This way, you can share files with people that don't even own dropbox.
When the sharing is done, just deleete the file from your dropbox.
The other great thing for people that run computers from multiple locations,
is that by installing dropbox on multiple machines, files are shared in
dropbox between their machines and devices.