You can use iMovie, iDVD, iPhoto or AppleWorks to create slideshows. I would recommend using either iMovie or iDVD.
The advantage of iMovie is that you can add and edit music, voiceovers, video, etc., you can tailor the length of each individual shot, you can use titles and transitions (the cross-fade works very nicely), and you can use the Ken Burns effect and other effects. In other words, it's a video that happens to be composed of still shots. You can then transfer it to DVD and create menus, etc. It's the most versatile tool.
The problem some people have with iMovie is that it reduces all images to 640x480 resolution. Some people want to preserve the high resolution of their images, depending on how they're going to view them.
iDVD slide shows are quicker and easier. You can add music but you can't edit the music or vary the length of each shot or apply transitions or effects. I haven't used iDVD's slide-show feature so I'm not sure if it reduces images to 640x480 or not. Also, my comments apply to iDVD2 - I'm not familiar with iDVD3.