One (Server) serves GIS resources as REST end points.
The other (Enterprise) does that and lets you wrap the services in out of the box web applications.
In terms of implementation:
Option one is several cups of coffee,
Option two is fill your thermos of coffee many times...
I am guessing you are looking at different versions of the software. Starting at 10.5.x, Esri rebranded their server product line as "ArcGIS Enterprise" (What is ArcGIS Enterprise?—ArcGIS Enterprise | ArcGIS Enterprise ). For 10.4.x and earlier, the name of the product line was "ArcGIS for Server" (What is ArcGIS for Server?—Documentation | ArcGIS Enterprise ).
The Server product was also called "ArcGIS Server" before being called "ArcGIS for Server" (which I always thought silly), before being named "ArcGIS Server" again, and then bundled as a component in "ArcGIS Enterprise". As Romeo well knew, the name does not alter the flower to which it refers (to extend that metaphor, Enterprise is a garden).