What is the difference between FIFO, LIFO, and Average Cost?

FIFO:First In First Out:

Assumption is stocks available are from The latest in with the correponding rates.Rates considered for  valuation are current.

LIFO:Last In First Out:

Assumption is Stocks available are from the earlier purchases made with the corresoponding rates,after making allowance for the consumption from the latest purchases.Under inflationary situations , these rates will pull down the profits.

Average cost is for each item, the avaerage of the total cost of purchase as divided by the total units purchased .This average is applied to the total units in stocks. Assumption is stocks contain mixture of stocks old and latest items.So the average cost(more so the weighted average cost ) irons out the positives and negatives of both the above FIFO and LIFO.