NGINX is much more then just a server like Apache, that's probably why it gained so much market share over the few last years if you trust the Netcraft server market share statistics.
NGINX has two advantages over Apache, first one is super fast, second one it's consumes less resources then Apache. Those are two important wins in two areas that really matter when having to decide which server to choose when getting started. Nginx is so fast because it's event driven. You may already have seen how javascript uses events and asynchronous loading of data. Nginx is similar to this, it emits a request, but doesn't wait all the time for a response keeping the connection open. Nginx requests something, then closes the connection and does something else. But in the request is a callback. So when the requested data is ready the nginx callback gets called and the response is send through a new connection to nginx. This is why nginx can handle such a lot of connections.
But as i said NGINX is more then just a server. The powerful configuration mechanism lets you setup NGINX to be a proxy server for one or multiple apache servers. You can use it as load balancer for multiple node.js instances or "just" use it as normal server to host your php website. If it's a php website you want to host.