An in-process subset of MicrosoftSqlServer.  Stand-alone applications can be deployed much more simply with an in-process database than with a separate server process that must be separately managed.  Microsoft finally has an in-process database that is a substantial and true subset of MicrosoftSqlServer rather than being a completely distinct code base with its own slightly wierd SQL dialect (JetDatabaseEngine).

''Does it require use of DotNetCompactFramework, or can it be used in environments not intended for WinCe?''

It is intended for both mobile and desktop applications.

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see http://www.microsoft.com/sql/editions/compact/default.mspx