Ok - simply put, a fork in the blockchain (the long ledger of transactions) can happen when two mining pools (as an example) find a bock at the same time.
Each block in the chain is made up of part of the previous block plus the batch of transactions.

for instance, the current block being 'mined' is #13550. this current block or records has three transactions in it. Let's say pool A and Pool B both find it to be a bonus block at the same time:

  Pool 'A' takes some information from that block (#13550) and adds the new transaction it just made - for finding a bonus block - smashes it together to make a new block - we will call that block #13551

  At the same time, Pool 'B' does the same thing - some stuff from block #13550 plus the transaction it just made for the block it found - smashes it together and puts it out as block #13551

these new blocks have to be verified (confirmed) several times. That's what the miners do, the crunch the numbers, make sure it all adds up - plus record checking stuff -  then passes it along.

So far, pool A being on the west coast doesn't know about transaction from pool B (maybe it's on the east coast?) and we now have two chains.

more in the next post