1. require() is a convenience function used to check conditions and throw an exception if a condition is not met.
2. Earlier versions of solidity (prior to 0.4.13) used the throw pattern to revert the state during an exception. It is deprecated now and will be removed in a future version:

// Older and deprecated method
if (msg.sender != owner) { 
    throw; 
}

3. Use the require() method to ensure conditions such as inputs are valid, or to validate return values from calls to external contracts: 

require(toAddress != address(0));
require(targetContract.send(amountInWei));
require(balances[msg.sender] >= amount);

4. Return an error message or a number corresponding to an error type using require():
   

require(condition, "Error Message");

5. The function will refund the remaining gas to the caller if an exception occurs.
6. require() uses the 0xfd opcode to cause an error condition. The 0xfd opcode is currently (EIP-140) mapped to the REVERT instruction.