Span.innerHTML returns text but Span.value returns undefined?

I have a function that populates a pages with something like this

<span id="span_title_'+result_no+'">'+title+'</span>

and then I have another function that has this:

      document.getElementById("span_title_"+which_table).innerHTML="asg";
alert(document.getElementById("span_title_"+which_table).value);

The strange thing is the first (innerHTML) call works perfectly, the second one, the alert, gives me "undefined"

Any idea why this is?