Imagine one day you're sitting along the coast of Northern England, taking a rest from farming in a bog, fishing, or whatever it was ancient villagers did up there back then. Chances are good you had a hard day of farming or catching fish and the end of the day was a welcome respite, even though you knew you'd probably have to go right back out and do the same thing the next day. But maybe you wouldn't, because Viking raiders were going to burn everything you love and there's nothing you could do about it.
That got real dark, real fast. Just like a Viking raid.
They were like today's special operators
Viking raids usually consisted of a small number of ships and limited manpower, headed for a very specific, small objective. They weren't out to capture towns or topple governments, they wanted food, booty, women, plunder, gold... you get the idea. The effectiveness of their raids hinged very much on their ability to surprise the opposition. They would move just over the coastal horizon, with their sails drawn down to mask their approach. Once inland, they would hit hard and fast, leaving before reinforcements could be brought to bear.
They weren't trying to sink ships.
You can't sell or reuse a sunken ship, after all. Though Viking naval combat was not very common, it happened. And like their land attacks, Viking longboats would swarm a target to overwhelm it, or they would attempt to ram the enemy in the open sea. Rather than have a distant naval battle, Vikings threw that doctrine out, preferring to move in close and kill the enemy crew with archers, hidden behind a hastily constructed shield wall.
In an age where tight formations and discipline in combat were all the rage, it was unlikely anyone expected a Viking horde to ambush their army as it marched through the woods. But here they were. Vikings used to lie in wait in the wooded areas along the roadsides, in order to get the drop on an enemy unit.
Adapting to the battle quickly.
Even the best plan can get tossed out the window once the sh*t hits the fan. The Vikings weren't perfect and would occasionally get their asses handed to them. On the occasion where that occurred, they adapted to the situation as quickly as they could. Once confronted by real opposition, raiders would take on infantry formations, especially the wedge, with berserks at the tip of the spear. They would then drive this into an enemy formation, negating the enemy's use of their archers or other ranged weapons.
Nothing was sacred. Sometimes literally.
These days, we talk about military norms that we all hold to be true – doctrine – as if it came from the gods themselves. Well, the Vikings didn't care much for your gods or your doctrine and pretty much flaunted both. They shook off the sacrilege of sacking religious sites because religious sites are where the best loot was kept. They shook off the doctrine of combat formations, fighting seasons, and times to do battle because that's when you were expecting them and it's so much easier to surprise you.
They wanted to get in close.
Many, many weapons of the middle ages were ranged weapons, designed to get into action at a distance and keep the enemy from smashing your squishy skull in. The longer one army could pummel another with arrows and boulders, the less likely their infantry or cavalry would die fighting. The Vikings, on the other hand, like the up-close-and-personal touch of smashing in your squishy skull and designed their battle tactics to get all up in your face, scare the crap out of you, and either kill you or make you run away.