Hard vs. Soft Forks
Consensus rule changes broadly fall into two categories: hard forks and soft forks. By definition, hard forks loosen or eliminate existing rules whereas soft forks only tighten or add new rules.1 A hard fork does away with presently existing validation checks, which automatically calls for the creation of a separate chain, independent of the original one. Soft forks, on the other hand, add new rules and validation checks, but make sure that new blocks will be backwards-compatible and accepted by clients, which haven't upgraded. In general, soft forks do not result in chains splitting from one another. Do keep in mind, however, that at a certain point of time, clients having the soft fork update might stop accepting pre-soft-fork blocks, essentially forcing old clients to upgrade.