The main difference is its architecture. How it was designed to handle HTTP request.
Apache:

Apache creates processes and threads to handle additional connections.
We can configure the server to control the maximum number of allowable processes. But if this is creased outside of server capacity, too many processes exhaust memory and can cause the machine to swap memory to disk, severely degrading performance. Plus, when the limit of processes is reached, Apache refuses additional connections.

Nginx:
Nginx handles the HTTP request asynchroneously and its event based.
It also do not create new process per request instead it has no of process defined (.e.g 4 total nginx processes) during start time and it creates threads out of it. Each of these processes is single-threaded. Each worker can handle thousands of concurrent connections. It does this asynchronously with one thread, rather than using multi-threaded programming.