They brought security. Before the Norman conquest there had been a three way war raging for the throne and the threat of Danish invasion loomed. Once the Normans took over they built around one thousand castle in 4 generations (including Colchester Castle, the largest surviving European keep) and succeeded in avoiding invasion by Denmark.

The Normans also centralised the country. The Domesday Book was the first census in England and gave King William a complete inventory of what the country owned.

Finally, the Normans introduced some highly impressive architecture, particularly in churches and colossal cathedrals. They also introduced their culture, a vital step in the creation of modern English culture.

I usually search for 'Norman' occasionally to chip in. Hopefully I can give a brief overview.

Firstly though Doomchicken7 is right in that the Normans brought further security, castle building, Norman or early Romanesque architecture and bureaucratic centralisation. If your interested to how far historians agree on the above changes then Marjorie Chibnal's the Debate on the Norman Conquest is a brilliant introductory work of the historiography.

Other changes the Normans made:

• Language - introduction of French vernacular (common speech) and Norman French for the nobility.

• Geo-politics - heavily militarised the border between Wales and Scotland. Later incursions and conquest into Ireland.

• Law - Royal Forests and law reform

• Church - In the early period Pre-Conquest the kings took a heavy interest in Church reform.

• Society and economy - archaeological evidence points towards an increase in population and the circulation and printage of coinage which is indicative of a growing economy.

You may want to look at an answer of mine to a similar question a couple of days ago which contains some recommend reading.