missing topic i cant understand..

i was learning and listening to chapter 5 inventory control and was really satisfied with the lectures till i decided to read my book to see if i missed anything and its under the chapter accounting for materials in my book but then the last few pages had these topics FIFO LIFO AVCO and periodic weighted average.. i understood the basic idea of either earlier stock is first served or later stock at that price but the examples confused me completely..i tried reading and understanding them but i couldn't quite understand them at all..is there anyway you could explain them to me?