Unix has many important and powerful features. Perhaps the three that have contributed most to its success are multi-tasking, multi-user, and build in networking.
Unix is a multi-tasking operating system. A user can perform more than one task at a time. These tasks or processes can be run in the background so that the user can continue with other activities. For example, a user could be sorting a file or making a calculation in the background at the same time they are editing a file.
Unix is a multi-user operating system. Many users can be using the same machine. This is directly related to multi-tasking since each user just has their own set of processes. The multi-user aspect allows groups of people to work easily together, sharing files and utilities.
Unix networking is built into the operating system. A diverse and powerfull set of networking tools have developed over the lifetime of Unix. Today some of these tools are becoming standards in non-Unix enviroments.