K-means vs. online K-means
Online k-means (more commonly known as sequential k-means) and traditional k-means are very similar.  The difference is that online k-means allows you to update the model as new data is received.

Online k-means should be used when you expect the data to be received one by one (or maybe in chunks).  This allows you to update your model as you get more information about it.  The drawback of this method is that it is dependent on the order in which the data is received (ref).