Links Woolwich and North Woolwich, and the North Circular and South Circular.
The free ferry service was instigated by Sir Joseph Bazalgette using powers granted in the Metropolitan Board of Works (Various Powers) Act 1885 and opened 1889. The ferries have been named after prominent London politicians.
The Royal Borough of Greenwich page is [1], the TfL page is [2] and the Wikipedia article is at [3]