What are servers for?
Servers are in general computers "optimized" or specialized on certain things. They generally don't have video cards or have very basic video cards because once they're started applications running on them don't need to show stuff on monitors, those applications run in background.
For example, the page you're reading now was retrieved from a server.
Your browser connects to a server at the other end which has the specific role of serving web pages to people. The server understands that you want to read this specific forum page, so it connects to another server that's specialized in databases and retrieves all the information that's required to build this page (like the individual messages, names of users, what you see under the nicknames in your case "zindan" , that you're from Sweden , that you have 458 posts as I write this) and then uses the processor in the server to build the page. Once the page is built, it's sent to your browser like a regular file download.
As the browser downloads the file, it reads it and decodes it and gradually shows you the page
Some servers (for example web servers, the servers that serve pages) don't need a lot of disk space but they may need a lot of memory, if a lot of people are viewing different pages of the website at same time.
Database servers may need a lot of memory AND may need a SSD because random data has to be retrieved from disk super fast - it would be pointless to use expensive SSDs on regular web servers.
File storage servers are usually optimized to serve static files (jpg images, css files, pdf documents) - these servers need a lot of disk space but in general they don't need a very fast processor.