I read the article about the difference between oplog and poll-and-diff, which takes an example of posting a letter, that articles explains a bit like this
poll-and-diff: this is like posting a letter to someone and going to post office regularly and checking whether we get the reply or not
oplog: We post the letter and asked postman to send post if we got reply
This is easy way to learn the difference between oplog and poll-and-diff.
Any body have the link to that article??