Most of the big players offer a suite of execution algorithms for big orders, as seen in this listing from Credit Suisse.
Very generally speaking, the algorithms will have a pedigree going back to volume weighted average pricing schedules, or perhaps to the famous paper by Almgren and Chriss. They have various modifications, including use of "unusual" order types, trades in dark pools etc., etc., along with parameters exposed to Credit Suisse's customers to allow control of timing, aggressiveness and so on.
Alex C. explained in the comments how this differs from iceberg orders, which are generally used as part of an algorithmically generated set of trades.