Longships are galley-like warships available to Scotland, Denmark, and now Friesland. They have a "boarding" attack, which is a fancy way of saying they attack at melee rather than at range. There is a known bug with longships and its former "upgrades" to the parent game's old ships, which will be noted later.
Trained at: Dock
Cost: 90 Wood, 30 Gold
Armor/Pierce Armor: 0/10
Special: +8 vs. other ships and lancers, +6 vs. buildings, +3 vs. rams.
The Longship in AoC:H is based upon the Galley, a standard arrow-firing warship in the parent game, Age of Empires II. Due to unknown causes, the game still believed the Longship upgrades to the old Galley's sucessors, the War Galley and Galleon, and used to upgrade the Longship for preset ages.
This would be a mere annoyance, and thankfully civs receiving the longship have no trouble (except when playing deathmatches - hence the warning against it), except that these other two, editor-only ships are bugged. They turn into Asian ships when attacking, and although they are powerful units, they freeze and make a single attack. The player must reorder the unit to attack in order for it to attack again. The bug appears to have been rectified in 1.72; War Galleys and Galleons are no longer available in the Scenario Editor.
The former Viking unique unit has been modified to serve a slightly different purpose: that of a "boarding" vessel. That is to say, it has a melee attack. It is now available to three civilizations, all of which were in Viking domain at one point or another, and all of which lack galleys. The longship is cheaper than a galley, and also is a very quick ship, useful in the scouting role.
King of the naval battle, longships can be of better use than a demolition ship - even against buildings - when used correctly, but contrary to popular opinion, they suck at sea-to-shore operations if the shore is well defended. In despite of their high pierce armour, they are easy prey to archers, like galleys, and still die to not only castles and towers, but also anticavalry units (as ships, camels and petards, demolition units similiar to demolition ships, were grouped together in the parent game).
Bottom line? Use 'em they way they were made: a replacement of the galley. Otherwise, that's a precious investiment lost.