Distribution Point is for distribution of software packages.
Management Points receives hw/sw inventory from clients and updates the clients with new policies. The policies tell the client what to do. For example turn on software inventory and install different software's from distribution points.
Secondary Site Server is used when you decide to install a secondary site. A secondary site is a site without a database. It receives all settings from the parent site and uses the same database as it parent. You primary use Secondary Sites to control traffic between offices with slow network bandwidth. You can control when data is allowed to go over the WAN and how much bandwidth it can use.
There are several more site roles you can install, for example Software Update Point. All with different roles in your site.
