CSS table default padding or margin

Today it's my first time I'm playing around with tables and I have noticing that the table and tr and td tags have a little space between them, like 1 px or so.

So here is my problem :

There is my code :

<table id="upload_box_container">
    <tr>
        <td class="border_bottom_1px">hi1</td>
        <td class="border_bottom_1px">hi2</td>
    </tr>
</table>

(upload_box_container - it's just background color and border color)

(border_bottom_1px - as it's name it only gives bottom border with 1px size)

and there is a picture of how it displays: http://postimage.org/image/16wz2ao78/

My question is

• why there is a space between the two bottom borders

• and why there is a space in the sides of the table (like padding) and the borders don't touch the table border

thanks.