The biggest difference is that nginx uses an evented I/O, and handles many connections per thread/process, Apache can only handle one at a time. This allows it

The biggest difference is that nginx uses an evented I/O, and handles many connections per thread/process, Apache can only handle one at a time. This allows it to to use far less memory per connection, and lets it perform reasonably under very high loads. It also has very low latency, even for small loads, and is relatively easy to install and configure on most platforms. nginx does an excellent job as a reverse proxy for applications, there are many configurations where nginx acts as a load balancer, and serves static content, and everything else is passed off to a real application. It's also useful if you're running on a tiny VPS with very little RAM. However, Apache has a few features that nginx lacks, like embedding languages into it like mod_php does, and per directory configuration, in that directory.