will i keep everything and whats the difference

I guess you mean will you keep all of your downloaded files? Where's your saved folder located? The default is C:\Documents and Settings\your username\Shared. You can check by going tools > options > saving...you'll see the location of your saved folder. As long as your saved folder is in a safe place (like the default) your downloads are safe too.

What's the difference? Difference in what? If you mean what's the difference between online & offline installers...using an offline installer, you download the installer to your computer & save it. Then install from there. Rather than installing over the net. There's less chance of corrupted installation if you use an offline installer.