InsiderFinance identifies block trades and displays them in the dashboard in the Type column as shown above.
A block trade is a large, privately negotiated order between two parties. They are often done outside of the open markets (i.e. not through public auctions) as their size can cause large fluctuations in volume.
Due to their large size and method of execution (i.e. private negotiation), block trades are almost always made by institutional investors and large hedge funds through investment banks and other intermediaries (usually a blockhouse).