What are these .dll's that I see embedded or referenced in web pages?

I'm researching how to build some tools in my website and as I'm looking around examples of similar tools in ot开发者_开发技巧her websites, I've noticed the source code referring to .dll files. I'm relatively new to web development, so I'm only accustomed to seeing references to script files. What are these .dll's that I see referenced in web pages?

Thanks so much in advance for your wisdom!

They are just server side code that will generate HTML (or JS/CSS/Images), similar to .php files or Java Servlets.