What is the difference between web api POST and PUT verb

we know that when we send data to insert then we use post and when we send data to update then we use put but if we want then we can do the reverse things.

i ask this question in sevaral forum like asp.net forum but one one can explian this reason with example rather every body saying put is idempotent. i was not satisfied to get their answer.

so if any one knows then can anyone explain the difference between web api post and put with example. also show me what does mean PUT is idempotent with sample code.

thanks