Batch, or offline, learning trains on all the available data at once and deploys a fixed model; to learn from new data you retrain, usually from scratch. Online learning updates the model incrementally as each new example, or small group of examples, arrives, so it learns from a stream and can discard data once used.
The simplest online learner is a running mean: if four values average 10 and a fifth value of 20 arrives, the new mean is 10 + (20 − 10)/5 = 12, without storing the first four. Online gradient methods likewise take one step, w ← w − η ∇L(w), per new example.
Don't confuse batch learning with batch or mini-batch gradient descent: a batch learner may train with mini-batches. Online learning suits streams and drifting patterns such as fraud, and its learning rate sets how fast it forgets the past. Its risk: bad input, from a bug or an attacker, flows straight into the model, so monitor it and keep a rollback.