Too Long; Didn't Read
Hard forks and soft forks change the rules of a protocol by reducing the set of transactions that is valid. Hard forks allow previously invalid transactions and blocks to become valid, so clients must upgrade their clients to stay on the hard-forked chain. Soft forks only really require consent from miners/validators (as even if users still use the old rules, if the nodes making the chain use the new rules then only only things valid under new rules will get into the chain in any case; hard forks require opt-in consent from users.