As a ship's boat the pinnace is a light boat, propelled by oars or sails, carried aboard merchant and war vessels in the Age of Sail to serve as a tender. The pinnace was usually rowed, and could be rigged with a sail for use in favorable winds. A pinnace would ferry passengers and mail, communicate between vessels, scout to sound anchorages, convey water and provisions, or carry armed sailors for boarding expeditions. The Spanish favored them as lightweight smuggling vessels while the Dutch used them as raiders. In modern parlance, pinnace has come to mean a boat associated with some kind of larger vessel, that doesn't fit under the launch or lifeboat definitions.
Pinnace attached to HMS London chasing a Dhow in 1881
"Name of Boat" attached to HMS London for chasing Dhows
With the introduction of steam propulsion came the steam pinnace. Coal burning warships were particularly vulnerable when at anchor, immobile until they could get a head of steam. Steam pinnaces were designed to be small enough to be carried by the capital ships they were allocated to and in addition to other duties were armed to act as picket boats.
One example of a ship utilizing many steam pinnace (ship's boats) was HMS London in Zanzibar while suppressing the slave trade in the region. "Slavery was legal in all Muslim countries, and HM ships could only become involved with slaving when it took place on the high seas. The boats of HMS London were kept at five minutes notice, ready equipped with water, salt pork, biscuits, arms, local currency and a small cask of rum. Manned by eight or nine sailors, with a midshipman or junior lieutenant in command, a boat was often away from the HMS London for two or three weeks, normally anchoring every night, the men off watch sleeping along the thwarts". 
^cf Knox, Dudley, ed. (1940). Naval Documents Related to the Wars With Barbary Powers, Naval Operations from 1802 to 1803II. U.S. Gov't Printing Office. pp. 267; 270. (examples: "[a]t 5 sent our pinnace alongside of a French Man of War (lying at Tunis) with a letter to Consul Eaton. . ."; "[a]t 8 the pinnace returned from the island, she found no bottom within 20 or 30 yards of the shore."; "[a]t 2 lower'd down our pinnace alongside of an American vessel lying in the bay. When the pinnace returned Lieu't Stewart gave us the following interesting news . . .")(extracts from journal of U.S. Frigate Constellation, Captain Alexander Murray, U S Navy, 6 Sept. 1802).