Server
A server is just a computer with an internet connection that is always turned on while running an application. This application is waiting to receive requests from clients. Once a request is received, a response with the corresponding data is send back. A server application that is responding with HTML documents and files related to the document is called a web server or web server application.
In theory, you could configure any computer that is connected to the internet to work as a server but in practice computers with specialized hardware are used. So instead of keeping a computer continuously running as a server at home, you can rent a server in a data center via hosting platforms, sometimes also called cloud providers.
In everyday language the vague term 'cloud' is used to describe a server and the applications that are running on it. In general the term 'server' is a bit muddy. When saying 'server' sometimes the computer is meant and sometimes the application running on it. So, it depends a bit on the context what is meant.
Some web servers always respond with the same web page for a particular URL. In that case, we talk about static webservers and web pages. But other web servers dynamically retrieve data from a database and include the resulting data in the web page before sending it to the client (optionally the data is processed in-between). For example, when you log in into an account for a social media platform, almost all the information you see is customized for your personal experience. Also, it is possible that the database application is running on a different computer than the web server application. There are many more use cases for servers as you will see.