Some algorithmic changes go completely unnoticed. However, the impact of a major algorithmic change can usually be seen quite quickly, though the change sometimes takes a few weeks to completely roll out. Algorithmic changes come in three forms:
Algorithm Update: The search engine changes certain signals of an existing algorithm.
Algorithm Refresh: The search engine re-runs an existing algorithm using the exact same signals as last time.
New Algorithm: The search engine adds a new algorithm to improve search quality. For example: Google Panda, Google Penguin.