The basis about library

Thanks very much, both of you.

Now, it seems I have a basic understanding.

Let me say this way, so in VHDL design file (I mean I create a design with VHDL file), the reason I put the library command like (Library altera_mf;)

This is because in my design, I need some entity that are not in my project, with the library command, Quartus can know where to find these entity because library give the path information, right?

IEEE library is a little different, it has data type and definition of operations (I don't know whether there are entities). I am also curious where the IEEE library physically exist in my computer.

Thanks.