SQL Server has two licensing approaches:
Server + CAL. With this license, every device that accesses the SQL Server directly or via a proxy needs a license. This can be cost effective if you have a fixed number of user devices. As internet users do not have device CALS this license can not be typically be used with a web server
Server + Core. With this license you license the server, and users do not need a CAL. the license is based on number of processor cores the host machine has. If hosted in a virtual machine, then the number of cores allocated to the visual machine, but a 4 core license is the minimum for a VM even if only using 1 core. Core licenses are sold in packs (number of cores licensed).
the difference between Standard and Enterprise is features and scaling. To decide between CAL and Core you need to run the numbers. If accessed by a web server in most cases you will need a core license.