There are two basic types of users on the LightRocket system: contributors and clients. Indeed you will see these two options on the registration page.
Contributors are users who will upload files into the system. Typically these would be photographers, staff members reporting from the field or indeed any individual who wishes to upload material to your archives.
Note too that any user you wish to make into a system admin will first need to register as a contributor. This is because admins need to have the ability to upload to the platform.
Client users are those may wish to create and share lightboxes or download files from your system. This could be members of your comms team, designers or media contacts seeking imagery to illustrate a story about your work.