A server is basically anything which serves something to something or someone. It could be a person, software or even hardware. Usually when talking about a server in a SD context, we mean a piece of software replying to network requests or a piece of hardware hosting such a piece of software.

What exactly a server is supposed to do is highly dependent on what kind of server it is. If a server is supposed to give pizza to clients in a restaurant, then it's their "work" to fetch pizza from the kitchen and carry it to a table (and not the client's^^)

It's the same for software. Server software has to accept a request and handle it. If the server should serve websites, then it has to use all information to load and build HTML, CSS and JS and send it to the client. If the client requests to delete a user, then the server has to check if the client is authorized to do so.

As for hardware, the server is supposed to be up and running all the time and run an application at all times. It's job is to deliver service uptime and performance.

As you can see from above examples, if you want to know the exact tasks of a server, you will have to be way more specific about what the server should do deliver, so that we can tell you what the server has to handle.

A server is just a computer. However, its hardware and software is a little different from the ones we use in our offices. A server has hardware designed to operate outside our normal PCs operations i.e its HDDs, RAM, CPU etc are somewhat made to 'endure' long hours of operations without breaking(and the hardware is expensive too). Servers also have stricter quality control, designed to do stronger work and stay long doing it. Ooh, and its hardware is optimized for any type of workload.

On the other hand, special server softwares allow for specific activities to take place, for instance, a web server will perfectly 'host' websites etc and we have many like email server, storage servers etc.

This does not mean that a normal PC can't serve as a server, when I was starting my business, I converted one of my old PCs into a web server and hosted my clients websites before I purchased an actual server. It served me for almost two years and I proudly still have it but hosts my internal accounting system.

Nowdays, there are so many things a server can do. But to sum it up, its just a PC.