Well, the <a> tag is a link on the page, but in this case an image is the link, because of the <img /> tag.
In the <a> tag, the "href" attribute is where the links actually brings you when you click on the image.
In the <img /> tag, the "src" attribute is the source of the image (where the image is located), the "alt" attribute is the text displayed when there is an error loading the image, and/or when you mouseover the image, and the "border" attribute image is self explanitory.
Do you understand it now?