The other users are correct, but since this question is so simple, i fear their explanation might be a tad to difficult (no offence OP, we all have to learn).
I'm going for an ELI12 explanation which is seriously simplified, but should (more or less) explain the difference between "block", "transaction" and "bitcoin".
Let's start
There is no such thing as a "physical bitcoin" (unless you're talking about those novelty coins, or artwork or something). When you get a bitcoin, it's basically an entry in our public ledger... So, basically, our ledger now contains a piece of text that says "sombody used a previous output noted in our ledger to fund one of the addresses of ukaddict with 1 BTC".
When the sender just created this piece of text (the transaction), it's usually just floating around in our network. Nodes receive this piece of text and put it in their mempool (literlly, they just put it in their memory). This also means the piece of text can be lost (and if it's lost, the network no longer knows the transaction, and your 1 BTC is gone)...
A miner's task is to take transactions (pieces of text) from their nodes' mempool and bundle them into a block. Think of one block as an excelsheet containing several thousands of transactions (unspent outputs being spend to fund addresses belonging to people). There's a lot of technical details involved in mining, but in this ELI12, we're not going to go into these details.
If a miner finds a valid block, it becomes part of the blockchain. He sends his block to the other nodes, and they save the valid block on disk... At this point, it's very hard to rollback your transaction, since it's no longer in memory, but it's actually written in a block that's part of the blockchain on the disk of thousands of nodes.
The blockchain is basically a string of blocks, think of it as excelsheets that are numbered: block0001.xlsx, block0002.xlsx, block0003.xlsx,... The sheets HAVE to be read in sequence, you cannot read block0009.xlsx before you've read block0008.xlsx. This is technically enforced. In reality, block0010.xlsx might contain a transaction that funds your address, and block0099.xlsx might contain a transaction where you use the unspent input from block0010.xlsx to pay somebody else (so it'll contain a record saying that you've spent the funds you got in block0010.xlsx to fund the address you've gotten from a seller somewhere).
Do realise this is an ELI12 explanation, it's far from complete (it's not even 100% correct, since it's simplified quite a bit).
