curl is generally used to interact with webservers, but can use other protocols. A google search for curl will give you examples -- https://curl.haxx.se/docs/httpscripting.html
It is most often used to download (or upload) files to and from the internet - from the command line. Or to test a connection to a server and/or if a specific protocol is working on a remote server. It is similar to the 'wget' command, except it is much more robust and can utilize more protocols
I just started learning Linux a couple of weeks back. I found an awesome article where you can find List of Linux Commands here. It's pretty helpful and very clear.