Is there a function to get the caption for an image in wordpress

If you are trying to get the caption while in a post you can echo it out inside your "the_post_thumbnail" tag.

<?php the_post_thumbnail();
echo get_post(get_post_thumbnail_id())->post_excerpt; ?>

You can also use the same method to show the image description. This is a little better feature in WordPress 3.5

<?php the_post_thumbnail();
echo get_post(get_post_thumbnail_id())->post_content; ?>

If you need to style the caption or description you can wrap it in a div like below.

<?php the_post_thumbnail();
    echo '<div class="myDiv">' . get_post(get_post_thumbnail_id())->post_excerpt . '</div>'
; ?>

Hope that helps.

Turns out captions are stored as post excerpts. So,

<?php echo $post->post_excerpt; ?>

will print out the caption if you are on the attachment image page (image.php in your theme) and inside the Loop.