I understand you are asking two things:
- What is an HTTP CLIENT?
This is any program/application used make communications on the web using the Hypertext Transfer Protocol (HTTP). A common example is a browser.
- What is cURL?
This a particular HTTP CLIENT designed to make HTTP communications on the web but built to be used via the command line of a terminal (command prompt).
If you perform a search for these topics, you will easily be able to find more in depth explanations about each.
solved What is an http client and what cURL?