What is an important difference between a soft fork and a hard fork?

Detailed explanation

For a consensus change, what matters is how the new set of valid blocks relates to the old rules. A soft fork tightens rules: updated nodes accept only a subset of what older nodes would accept. Older nodes can therefore keep following the new chain even though they do not fully check the new restrictions.

A hard fork expands or changes validity rules so older nodes reject at least some blocks valid under the new rules. If both rule sets remain in use, permanently separate chains can emerge. Hard does not automatically mean aggressive or bad; it describes a lack of backward compatibility.

A soft fork is not automatically risk-free either. Unupdated nodes do not check the new rule themselves and rely on enough relevant participants enforcing it. Activation and coordination are therefore central issues. Segregated Witness and Taproot, for example, were introduced as soft forks. The terms describe technical compatibility, not popularity or quality.