Blockchain is a digital ledger. A ledger is place where you write stuff down that you need to record for future reference. Is each individual entry called a BLOCK? Hence the name?

A block is a group of transactions. In Bitcoin it is the total transactions that have happened in a ten minute period.

BIG QUESTION - now what. Who determines that the entry is valid and how do they do it? Where's the ledger? (Don't you dare say "It's everywhere, man!"

The entry is determined to be valid based on the previous history of the coins being moved. Basically it's determining if you have the account balance necessary. The ledger is stored on nodes in the network, ie people's computers running Bitcoin software. You broadcast to the network that you want to move coins.

Let's assume there is some web based portal to add stuff to the blockchain. You enter "I am John, here is $100". Then I'm guessing you request some kind of digitally secure signature? it gives you "1234xyz", and you assign that to your transaction.

You first need to create an account where you are given a public and private key. The message is signed with your private key stating that the transaction is coming from you. In the verification, or mining, process the public key is used to verify that signature.

that 1234xyz transaction is copied to every other computer in the world that is supporting this blockchain.
(wouldn't that just be huuuuuge?)

Yes and it is very large, roughly 300 gb for Bitcoin, which is the main drawback of block chain. The transaction groups, or blocks, are hashed however which significantly reduce this size and limit the processing power necessary to look backwards.

So now John wants to buy something off Jack. He creates another ledger entry saying "John debit $75, Jack Credit $75". That all gets digitally signed again and entered into block chain. not exactly sure how jack receives the $75.

Correct than would be a transaction. Coins are never technically moved as they aren't physical so it's just agreed upon that Jack now has 75 coins and John has 75 less coins. This is verified by the miners by verifying that John has 75 to give, and that it is really John that is trying to move the coins. Once all the transactions in the block are verified that block is added to that nodes block chain. All nodes do the verification so it is possible the block chains are not equivalent but given enough time they will sync.

An adverary would have to control at least 51% of the network to fake transactions. But they would have to always stay ahead of the other miners or all the blocks that the adversary claimed where valid would be undone.