How can we submit a form without a submit button?

The main idea behind this is to use Java script submit() function in order to submit the form without explicitly clicking any submit button.
You can attach the document.formname.submit() method to onclick, onchange events of different inputs and perform the form submission.
you can even built a timer function where you can automatically submit the form after xx seconds once the loading is done (can be seen in online test sites).

In addition to what marwa khalaf suggested, you can also submit data back to the server without leaving the current page using ajax calls.
If you use jQuery that would be pretty simple using $.ajax and set the type to "POST".
See this link for more details http://api.jquery.com/jQuery.ajax/

a simple way by using image button it have the same effect as the submit button

using javascript submit()

Send Form function submit_frm(){ ob=document.getElementById('frm'); ob.method='post'; ob.action='page_name'; ob.submit(); }

div , image, span ->> onclick ->> AJAX Submit or normal javascript submit

using some js or jQuery.

You can use ajax for that like on change you can fire $('xyz').change(function() { $('form').submit(); });