https://blog.ethereum.org/2014/05/06/daos-dacs-das-and-more-an-incomplete-terminology-guide/
Blockchain
A blockchain is a distributed computing architecture where every network node executes and records the same transactions, which are grouped into blocks. Only one block can be added at a time, and every block contains a mathematical proof that verifies that it follows in sequence from the previous block. In this way, the blockchain’s “distributed database” is kept in consensus across the whole network.
Security
Individual user interactions with the ledger (transactions) are secured by strong cryptography. Nodes that maintain and verify the network are incentivized by mathematically enforced economic incentives coded into the protocol.
Decentralization
On traditional server architectures, every application has to set up its own servers that run their own code in isolated silos, making sharing of data hard. If a single app is compromised or goes offline, many users and other apps are affected. On a blockchain, anyone can set up a node that replicates the necessary data for all nodes to reach an agreement and be compensated by users and app developers. This allows user data to remain private and apps to be decentralized like the Internet was supposed to work.
Ether
Ether is the cryptocurrency of the Ethereum Blockchain. It is a necessary element -- a fuel -- for operating the distributed application platform Ethereum. It is a form of payment made by the clients of the platform to the machines executing the requested operations. To put it another way, ether is the incentive ensuring that developers write quality applications (wasteful code costs more), and that the network remains healthy (people are compensated for their contributed resources).
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Smart contracts
Nick Szabo 1994 article
Security
http://www.dejavusecurity.com/blog/2015/7/23/deja-vu-security-assists-in-ethereum-release