JQuery.attr():
Get the value of an attribute for the first element in the set of matched elements.

JQuery. Prop():
Gets the value of a property for the first element in the set of matched elements.

Reference MSDN:
for a checkbox (jquery 1.6+)
<input id="check1" checked="checked" type="checkbox" />

.attr('checked')     //returns checked
.prop('checked')     //returns true
.is(':checked')     //returns true

Prop() method returns Boolean value for checked, selected, disabled, readOnly..and so on while attr returns defined string. So, you can directly use .prop("checked") in if condition. SelectedIndex, tagName, nodeName, nodeType, ownerDocument, defaultChecked, and defaultSelected..and so on should be retrieved and set with the .prop() method. These do not have corresponding attributes and are only properties. .attr() calls .prop() internally so .attr() method will be slightly slower than accessing them directly through .prop().