input field, only numbers jquery/js

Try this:

$("#num").keypress(function (e){
  var charCode = (e.which) ? e.which : e.keyCode;
  if (charCode > 31 && (charCode < 48 || charCode > 57)) {
    return false;
  }
});

Values 48 through 57 represent the digits 0-9.

Never do this. A user can update a textbox without pressing the key. He can copy paste, drag. some text.

Also this will be irritating to the user.

Just display a label nect to the filed saying that this accepts only numbers. And then

Validate your code at submission

Comparing to the current best answer this code is more user-friendly - it allows usage of arrows, backspace, delete and other keys/combinations:

// Ensures that it is a number and stops the key press
$('input[name="number"]').keydown(function(event) {
    if (!(!event.shiftKey //Disallow: any Shift+digit combination
            && !(event.keyCode < 48 || event.keyCode > 57) //Disallow: everything but digits
            || !(event.keyCode < 96 || event.keyCode > 105) //Allow: numeric pad digits
            || event.keyCode == 46 // Allow: delete
            || event.keyCode == 8  // Allow: backspace
            || event.keyCode == 9  // Allow: tab
            || event.keyCode == 27 // Allow: escape
            || (event.keyCode == 65 && (event.ctrlKey === true || event.metaKey === true)) // Allow: Ctrl+A
            || (event.keyCode == 67 && (event.ctrlKey === true || event.metaKey === true)) // Allow: Ctrl+C
            //Uncommenting the next line allows Ctrl+V usage, but requires additional code from you to disallow pasting non-numeric symbols
            //|| (event.keyCode == 86 && (event.ctrlKey === true || event.metaKey === true)) // Allow: Ctrl+Vpasting 
            || (event.keyCode >= 35 && event.keyCode <= 39) // Allow: Home, End
            )) {
        event.preventDefault();
    }
});

Notes: The event.metaKey === true is required for Mac users (thanks RyanM for noticing this). Also if you uncomment Ctrl+V sequence you will need to write additional code for checking pasted text (disallow non-numeric symbols).