Retrieve And Pass Information To And From HTML Forms

This question may appear opposite of most, but I'm learning JS using a book and I can't understand why/how this particular code works. I'm working on simply retrieving and passing information to and from html forms. This is a very simple script that will change what is selected in the drop down box depending on a button push.

Here is the script:
Code:
function flip(pizzatype) {
if (pizzatype.value == "Veggie Special") {
document.forms["pizzaform"].topping.value = "veggies";
} else if (pizzatype.value == "Meat Special") {
document.forms["pizzaform"].topping.value = "meat";
} else if (pizzatype.value == "Hawaiian") {
document.forms["pizzaform"].topping.value = "hampineapple";
}}

And here is the form's html:
Code:
<form id="pizzaform" action="#" onsubmit="return false;">
<p>
<input type="button" name="veggiespecial" onclick="flip(veggiespecial)"
value="Veggie Special"/>
<input type="button" onclick="flip(meatspecial)" name="meatspecial"
value="Meat Special" />
<input type="button" onclick="flip(hawaiian)" name="hawaiian"
value="Hawaiian" />
</p>
<p>Main Topping: <select name="topping">
<option value="cheese" selected="selected">Cheese</option>
<option value="veggies">Veggies</option>
<option value="meat">Meat</option>
<option value="hampineapple">Ham & Pineapples</option>
</select></p>
</form>

The way I see it is the button onclick will call the flip function and pass it a var, for example "veggiespecial". I understand the code that will perform the actual change on the drop down menu. What I don't understand is the if statement.
if(pizzatype.value == "veggiespecial")...
Shouldn't it be just (pizzatype == "veggiespecial")?
Why does it have to be pizzatype.value? Where the heck does the .value come from???