Ethereum — is a kind of cryptocurrency, based on the blockchain technology. Ethereum at the time of writing is the second most popular currency after Bitcoin. If you like to go deeper then read "Ethereum" and "Blockchain" on Wikipedia, where you will find the most detailed explanation of the concepts. Ethereum Project website - ethereum.org
One of the main differences of the Ethereum compared to other currencies (for example, Bitcoin) is the support of smart contracts. In simple words, a smart contract is the ability to perform certain predefined actions during payment processing. For example, you can program the following behavior when the funds are received: check the payment amount, save information about the payer, send a confirmation that everything went well. The possibilities of smart contracts are enormous. If you want to know more, check the article "Smart contract" on Wikipedia.
If you are already interested, then be sure to read our articles "How to create ETH wallet?" and "How to buy ETH?". You are almost ready to enter the new age of digital currencies and get both pleasure and profit from it! ;). So much is already available for Ethereum right now.