Also, I have bad news for you if you are a web developer: unless all you're serving up is HTML (either static or generated on the server[1]) you are developing a multi-threaded app - it just happens to be the case that the threads are running on different machines. Race conditions between client and server code are a thing, so you'd do well to understand the concepts of multi-threading.

[1] And even in this case you're probably still multi-threaded, although in most cases it won't feel like it because your server side threads won't share state.