To get an operation done in Ethereum, the sender of the transaction must specify a gas limit before they submit it to the network & a gas price: 

• The gas limit is the maximum amount of gas the sender is willing to pay for this transaction. If an operation has LOW gas, ...

Ether is made of a blockchain (an immutable database) and a world decentralised computer (EVM), a virtual machine in which all the smart contracts function in Ethereum. 

The EVM is a simple yet powerful Turing Complete 256-bit virtual machine. Turing Complete means that given the re...

A smart contract functionality needs to be 3 things: 

Deterministic: it gives the same output to a given input every single time.

Terminable: contracts by definition, must be capable of termination in a given time limit.

Isolated:

If bitcoin is digital gold, Ethereum is alike the power grid, powering an ecosystem of worldwide decentralized applications. 

The gas is the metering system for the consumed power. And ETH is the currency used to pay for running the software and maintaining the network. 

When two numbers are added a million times in Ethereum it costs ~$26.55 in fees. On Amazon AWS system it costs $0.000000066.

This means that computation in Ethereum is 400 million times more expensive!

However:

• A well-written contract would likely move such...

If an operation runs out of gas, then it is reverted back to its original state like nothing actually happened, however, the operation generator must STILL pay the miners the fee for their computational costs and the operation gets added to the blockchain (even if it has not been...

Bitcoin created a decentralized monetary system which can transfer money from one person to another. That only allowed for monetary transactions, there was no way to add conditions to those transactions: 

Alice can send Bob 5 BTC, but she couldn’t tell Bob that he will ...