I wanted to clear up the definition of Apache:
Apache IS the HTTP server. So, when you use Firefox / IE / whatever to view your site, your web browser is establishing a connection to apache (on port 80) which then services your request by giving you images and generating the content (whether the files are just pulled from the disk or generated dynamically). Apache is very flexible and on the servers here and many other places, can also use the programming language PHP (along with others? perl? python? probably, I don't use them specifically) to generate the dynamic web page.
Apache is just a program like any other. So if too many requests come in and it can't handle it (whether the hard drive isn't fast enough or maybe the internet connection isn't big enough) then you'll notice slow connections or dropped connections. That's server load, the work that Apache is doing.
There's also of course the load on the server from the database service (mySQL) and the FTP server.