Answer from cs61c-ep (Hao Wen Vivian Chong 15826235) for Question 1
A virtual address is an address that corresponds to a location in virtual space and is translated by address mapping to a physical address when memory is addressed. A physical address is the corresponding address in main memory. A virtual address is like the title of a book, and the physical address would be the location of the book in the library. To go from a VA to a PA, the virtual page is looked up in the page table, and if the valid bit is on, the page table supplies the physical page number.