So it's kind of tricky to answer this one satisfactorily just because the period and the region is so vast, and because a lot changes during it. Just in England, the differences in weapons and tactics between 1000 and 1100 are striking.

I answered a question about Early Medieval warfare in England here, and discussed the centrality of the shield wall to combat, not only for the English, but also for the Danes. Fighting with spears in a shieldwall was the foundation of much early medieval combat, around which skirmishers, archers and later cavalry played a role. At the Battle of the Standard in 1138, for example, we can see the devastating effect of the English archers - protected by a shield wall - on an unsupported Scottish charge.

Stirrups start to reach Europe in, roughly, the 8th Century and revolutionise cavalry warfare. According to the Anglo-Saxon Chronicle, a Viking army captured a quantity of horses from the French in the 890s and briefly 'became a mounted force' before losing them all to the English in a disastrous defeat outside Rochester the following year. In 937 we see the beginning of an English flirtation with cavalry when mounted Wessex troops are used to ride down fleeing Vikings following the Battle of Brunanburh to great effect.

The real change, however, comes on the continent. In the 950s, Otto I develops his army of loricati, heavily armoured cavalry who specialise in high-impact shock charges, with enough armour to stick around in combat afterwards. Essentially, knights. These are used to devastating effect in the Battles of Lechfeld and Recknitz in 954, absolutely crushing Magyar and Wendish armies respectively and almost overnight ending the scourge of Magyar raiding which had repeatedly predated Germania, Lotharingia and even Eastern France for decades. Heavy cavalry wasn't enough to win battles alone - the English shieldwall famously resists repeated Norman charges at the Battle of Hastings - but they are an incredibly potent weapon which utterly change the strategic map and the ways in which armies can control territory. Surviving records from the First Crusade in particular show the terrifying impact of heavy cavalry charges against enemies largely unprepared to effectively counter them, with small Frankish forces purportedly routing much larger Turkish forces on a number of occasions through repeated mounted charges.

For some brief reading, I would suggest Strickland's Anglo-Norman Warfare, Lavelle's Alfred's Wars, Charles-Edwards After Rome and Bartlett's The Making of Europe.