An Ethereum account is really a combination of things:

• Public address - public representation of an account, derived from public key. eg. 0xCD2a3d9F938E13CD947Ec05AbC7FE734Df8DD826
• Public key - publicly available cryptographic key for verifying authorship of signed messages and transactions.
• Private key - secret cryptographic key for signing messages and transactions and decrypting data signed with the corresponding public key.
• Balance of ETH - the current amount of Ether held by the account.

Ethereum account balances are bounded by the network they were created in - so, if you create an account on the Ropsten testnet you can only use that account's ETH in transactions on Ropsten. The same goes for the 'mainnet', which is the live, production network.

Accounts are created programmatically using cryptographic algorithms. See StackExchange for a good, high-level introduction to these concepts.

There are numerous services that aim to augment this process with useful interfaces and tools, like MetaMask and MEW. Accounts created in this manner are often branded as 'wallets'.

You can also create accounts via Mist and on local test networks, such as Remix or Ganache (aka TestRPC).