How does USB differ from Serial port

The holes are a different shape...

Seriously though, they are different in many ways:

Read this: Universal Serial Bus - Wikipedia, the free encyclopedia
Now read this: Serial port - Wikipedia, the free encyclopedia

Now you understand many of the differences.

From my experience, USB is said to be faster than serial. We can't tell but the computer can. My printer came with serial. But because of my office set-up I needed a longer interface cord from the PC to the printer. I went shopping for an extension and found that coupling two together would make it even slower and the cord would have cost almost as much as a USB that was already long enough. For me that was a no-brainer. I go with USB any time I have the option.