Cart
From Shopify Wiki
The liquid variable cart represents the shopping cart and has the following attributes:
Contents |
cart.item_count
Returns the number of items currently in the shopping cart.
Example usage:
There are currently {{ cart.item_count }} items in your cart
cart.items
Returns all items in the shopping cart. Each one is of the type Line_Item. You will want to iterate through the return value (see example)
Example usage:
<ul>
{% for item in cart.items %}
<li>{{ item.title }}</li>
{% endfor %}
</ul>
cart.requires_shipping
Returns true if there is at least one item in the cart that requires shipping, and returns false otherwise. 
Note: to know if a particular item in the cart requires shipping, look at its requires_shipping property using line_item.requires_shipping.
cart.total_price
Returns the total of all the prices added up in your shopping cart.
Example usage:
The total of your cart is: {{ cart.total_price | money }}
cart.total_weight
Returns the total weight of all items in the cart combined. Weight is always returned as grams. Use weight or weight_with_unit from the filter library to display the actual weight in your preferred unit system.
Example usage:
The total weight of the items in your cart is: {{ cart.total_weight | weight_with_unit }}
cart.note
Allows you the option of adding a note field to your checkout template. Usage of this feature is very flexible. The general idea is that you simply define an input field named “note” in the form that submits to ”/cart” in cart.liquid. Here is an example:
<p>
<label for="notes">Extra notes or special instructions</label>
<input type="text" id="notes" name="note" size="60" value="{{ cart.note }}" />
</p>
When the order is submitted you will be shown the note at the top of the order details page in your admin area. You can check out this blog post for additional examples and explanation: http://jadedpixel.com/2006/8/18/attach-notes
cart.attributes
The attributes property is similar to the note property above in that it is an optional field you can add to your shop’s checkout form. Simply define an input field named “attributes[]” and it will be captured automatically and displayed on the order detail page in your admin area. So, for example, you wanted to capture the length of necklace a customer wanted you could do this:
<label for="wrapping">Gift Wrapping:<label>
<select name="attributes[gift_wrapping]" id="wrapping">
<option value="none"{% if cart.attributes.gift_wrapping == "none" %} checked="checked"{% endif %}>None needed...</option>
<option value="wrapping"{% if cart.attributes.gift_wrapping == "wrapping" %} checked="checked"{% endif %}>Gift Wrapping</option>
<option value="box"{% if cart.attributes.gift_wrapping == "box" %} checked="checked"{% endif %}>Gift Box</option>
</select>
Again, a visual example of this technique (and how to use a checkbox input for an attribute) is available at: http://jadedpixel.com/2006/8/18/attach-notes