Ripple: what’s the difference between “block” and “ledger” I have heard many times that Ripple changed the concept of Blocks for Ledgers.

What would be the difference between a bitcoin block and a ripple ledger?
Bitcoin

In bitcoin, there is no concept of accounts, users, balances and payments. The bitcoin blockchain just stores the history of all the transactions that have happened. Each block contains transactions that were mined in that block. A particular block does not contain any details of the transactions included in the previous blocks (just the previous block header hash which links it to the previous block). When someone says that his wallet has a balance of 1 BTC, it only means that his wallet contains private keys that satisfies the unlocking condition (signature) to spend that 1 BTC. For example, to get your balance related to the private key, you will need to scan across all the blocks that have been mined and find out the transactions that can be unlocked with your private key signature (SPV nodes do this by connecting to a full node and use bloom filters, but that is a matter of discussion for other day). 

Ripple

Ripple follows a ledger mechanism. By this we mean that unlike bitcoin blockchain which stores only the transactions, Ripple store account info such as the balances. So the ledger is a point in time snapshot of all the balances held by all the addresses. Each ledger has a version which is progressed by 1 as the ledger gets updated. If you want to get say your account balance, you can just download the latest ledger and need not download the previous versions of it. (You might only want to download the previous versions of the ledger in case you want to audit the ledger)