A block explorer is a service that allows you to easily view and verify information about the blocks, addresses, and transactions created by Bitcoin. It simplifies transactions by saving Bitcoin users the hassle of running the Bitcoin client to verify the exchanges. Blockchain.info is a block explorer.
A block is a record of some or all of the most recent Bitcoin transactions that have not yet been recorded in any prior blocks. An analogy could be to the individual pages of a recordbook or a stock transaction ledger. New blocks are added to the end of the blockchain, and never changed or removed.
A transaction shows coins which have been sent between one of more bitcoin addresses. Often transactions include an additional "change" output.
Address tags are identifiers known to you only. They help you to manage your addresses efficiently.
Public notes are embeddable short messages during the transaction. An example of a public note can be seen below:
You can embed a public note when doing a transaction with Blockchain.info. Simply input your message in the column below when sending a transaction.
The taint is the percentage of funds received by an address that can be traced back to another address. An example of a taint analysis is: https://blockchain.info/taint/1dice6GV5Rz2iaifPvX7RMjfhaNPC8SXH