The objective of designing a site is to ensure that when the users open up the site they see the information in a format that is easy to read and relevant. This is further complicated by the fact that users now use a large variety of devices with varying screen sizes and resolutions thus forcing the designer to take into consideration these aspects when designing the site. They need to ensure that their site comes up correctly in different browsers (cross-browser), different operating systems (cross-platform) and different devices (cross-device), which requires careful planning on the side of the developer.
The roles are ordered according to overlapping skills. A front-end developer will typically have a good handle on UI/Interaction design as well as back-end development. It is not uncommon for team members to fill more than one role by taking on the responsibilities of an over-lapping role.