A delta is defined as “a triangular tract of sediment deposited at the mouth of a river, typically where it diverges into several outlets” by Google’s define function. (It’s also an airline company, but that’s neither here nor there for this answer.) An estuary, by the same dictionary, is defined as “the tidal mouth of a large river, where the tide meets the stream.” A delta is actually listed as a synonym for the word.
However, there is a crucial difference here.
Just as the previous answer pointed out: the delta is the sediment, while the estuary is the water itself. You can look at the Mississippi river and where it enters the Gulf of Mexico for a good example. All the little out branches are the estuaries, while the land between are deltas.