You can place the users into different 'Groups' with the groups having different view permissions on folders.
Each group's permissions are set in Admin -> Groups. To bring up the group's view/download permissions, click on the link 'folders' next to the desired group. In this area, you can allow one group the ability to only view assets, and another group the ability to download, for example.
This means that you can have an asset in the system that some people are allowed to view, some people cannot view, and some other people can actually download.
You can add users to groups in the Admin -> Users section.