Assuming this example HTML:
<input type="text" name="email" id="email" />
<input type="text" name="first_name" id="first_name" />
<input type="text" name="last_name" id="last_name" />
You could have this javascript:
$("#email").bind("change", function(e){
$.getJSON("http://yourwebsite.com/lokup.php?email=" + $("#email").val(),
function(data){
$.each(data, function(i,item){
if (item.field == "first_name") {
$("#first_name").val(item.value);
} else if (item.field == "last_name") {
$("#last_name").val(item.value);
}
});
});
});
Then just you have a PHP script (in this case lookup.php) that takes an email in the query string and returns a JSON formatted array back with the values you want to access. This is the part that actually hits the database to look up the values:
<?php
//look up the record based on email and get the firstname and lastname
...
//build the JSON array for return
$json = array(array('field' => 'first_name',
'value' => $firstName),
array('field' => 'last_name',
'value' => $last_name));
echo json_encode($json );
?>
You'll want to do other things like sanitize the email input, etc, but should get you going in the right direction.