Hola,
Muy básicamente:
Código PHP:
<html>
<head>
<script type="text/javascript">
function AddProduct(product){
var ta = document.getElementById('products');
ta.value = ta.value + product + '\n';
return true;
}
</script>
</head>
<body>
<form>
<textarea id="products" cols="20" rows="10"></textarea>
<input type="button" value="New product"
onclick="AddProduct('New Product')" />
</form>
</body>
</html>