Printable VS. Online Form
On paper, there is a difference between filling out a form. You can't make mistakes with a pen or marker, you have to turn the page, your hand can't get tired, you cannot save it and enter it later, and you cannot make multiple copies without wasting paper. On the computer, there is no difference between filling out a form online and on paper.
The most obvious difference between the two would be the convenience of filling the form online and printing it off as opposed to filling the form by hand and then printing it off. Other differences might include the ability to save the form and return to it later, as opposed to filling the form and then having to start it all over again, or the ability to use a PDF as a guide to fill the form as opposed to writing everything out on the form itself.