Ledgers are collections/running spreadsheets holding records of crypto transactions. There are many copies of one ledger in cryptocurrency. Because there are so many copies of the same thing, every transaction you make is recorded independently on each copy. This way, if someone decided to hack a transaction, even if 99% of the ledgers say one thing, and one says another, the system knows that someone violated the rules of the system, and prohibits this from happening.

Blockchains are chains of blocks formed in a direct line (literally!) and are forms of digital ledgers, which many people use. Blockchains were largely adopted when bitcoin came around, which is why they are often associated with bitcoin. Blockchains essentially track data changes over time, and retain history for these changes. This means that if changes are made, the blockchain will add a new block exhibiting this change, and won't make any adjustments to pre-existing blocks. Unlike block chains, ordinary ledgers are subject to being hacked, tampered with, and are pretty vulnerable.