Very little would be done on the internet without the help of servers, they're like the man behind the machine for computers and hardware.
A server is normally a program or device that makes other programs or devices work, these devices or programs are called 'clients'. the main server making these programs or devices work will do so by sending data or resources to multiple programs or devices to enable them to function properly, for example if you play video games then for you to play online you must establish a connection to a server in order to play online.
Servers are extremely convenient because of how a single server can serve multiple clients at once, meaning if powerful enough, one server can send data to hundreds of thousands of people at once! This is particularly effective in places like huge companies like Microsoft, online multiplayer games, or even apps on your phone.
Having only few servers can cause issues though however, as servers can sometimes become overloaded with users, which can cause them to crash, slow drastically or even shut down temporarily, which can cause companies massive loss in profit or in the case of online gaming, nobody being able to play the game.
Though database and game servers are the most common types of servers, there are other types of servers that help in industry as well, these include file servers, which allows for shared access amongst multiple platforms for files, mail servers, which transfers mail across platforms to other clients (think E-mail), print servers, which allows devices to connect to printers wireless, and application servers, which helps web servers run a lot more smoother (we'll get to web servers in a minute)
Finally, as said above, there is the most important server of them all. The Web server, which is the server that works the entire world wide web (the entire internet!), this server is what you're using right now to read this!