What are C++ Libraries?

[LeInno1]

Can someone explain to me what libraries are and how to find the functions to call them out and how do you find this library? Because I don't understand how programmers are able to call a lot of functions within the library off by memory. I'm sorry if this seems like a stupid question. I'm pretty much a newbie for C++!

[giblit]

After you use something enough it will become second nature. Though we still often times read references.

http://www.cplusplus.com/reference/
http://en.cppreference.com/w/