This is a fun one! There is technically a difference, but it is so subtle that in almost all cases you can use these words interchangeably, with the exception of 'relentless'. Ruthless This comes from an old English word 'ruth', meaning a feeling of pity (we don't use it any more, no one will understand you if you say it!). So this word means 'without pity'. Someone who does terrible things to others without any thought for their feelings would be a 'ruthless' person, or their actions may be 'ruthless'. "Stalin was a ruthless dictator who caused much suffering." "Stalin governed his people ruthlessly." Heartless Similar to above, but a softer meaning - when you say someone has a heart, you are saying they are kind and considerate. So heartless people are unkind and don't consider the misfortunes of others! It's very similar to ruthless, but you don't really use it as an adverb much. "He doesn't care how badly she is suffering. He is a heartless man." Relentless To 'relent' is to show mercy and to be less severe than one is being already. Someone who is 'relentless' is someone who never gives up and doesn't show mercy. As an adverb, it can be used to describe a concerted effort that never slows down. You can also use this for non-living things, like a really bad storm. This is the only word in your list which doesn't necessarily have a negative meaning - it just means to not reduce the effort you are making. "Bruce Lee relentlessly trained his body every day." "The sandstorm raged relentlessly." Cruel This word describes someone who does bad things to others willingly to make them suffer. It is like a more intense version of the word 'bad' for describing people. "He was very cruel to his children." "He is a cruel man."