Modal windows come in 2 differing varieties, a straight forward pop up, that can contain text or images, triggered via button or link and a timed modal that will appear "X" seconds after the page has loaded, configurable via Javascript.
Probably by the time you have read this text the timed modal will have appeared. You can configure the timed modal to appear a set amount of time after the page has loaded via a simple Javascript setting.
Lightboxes, like modals come in 2 different varieties, a single image Lightbox, and a gallery option. The images opposite are single Ligthbox images.
When you click on one of the images opposite you will see its possible to shift to the next image using the arrow navigation, effectively turning your Lightbox into an attractive gallery.