The blockchain is the list of all solved blocks, essentially the ledger of all transactions completed in the Bitcoin network. Each block contains (among a few other things) a list of transactions.
A blockchain is a decentralized ledger - or a database. And data is verified collectively. A ledger is simply a database and by itself, has no relation to decentralization. In other words, a blockchain is a particular type of ledger, one that is decentralized (generally speaking, as centralized Distributed Ledger Technology ledgers also exist.)