Microsoft Dynamics NAV started its being as Navision, in about year 1985. It has changed names many times, after being bought by Microsoft.
"Navision Financials", "Navision Attain", "Microsoft Business Solutions Navision Edition" were one of those names. Now it is called "Microsoft Dynamics NAV".
What does it do? Helps small and medium businesse with customer relationship management (CRM), supply chains, finance, manufacturing, analytics and electronic commerce in. It can be "extensively customized" as customers have full access to the business logic source code.
Microsoft bought Navision A/S (that was merged Navision Software A/S and Damgaard A/S) in summer 2002. They also bought Axapta, similiar product developed by Damgaard A/S.
MS rebranded those products and released them as Microsoft Dynamics NAV in autumn of 2004. Microsoft will probably release Microsoft Dynamics NAV 5.0 in 2007.