What exactly is an mxd file, and how is it related to a database? Can a single database have multiple mxd files?
.mxd file specifies one particular way of rendering some data. The
.mxd file references your data (by file name, by IP address, by URL, etc, depending on the data). But it does not contain the data.
In the ArcGIS world, an
.mxd file is often simply called "a map". (The acronym "MXD" stands for "map document".)
And yes, you can create as many different maps (
.mxd files) as you want, each of which could reference the same data (whether it's a database or something else), but display it differently. (Or display it the same.)
An mxd file is simply your map.
It contains the links to your geodatabases that the current mxd is using, plus any settings that you configured in the mxd (such as symbology, extent, etc).
Multiple mxds can all access the same geodatabase without problems.
Here is a quick reference - http://wiki.gis.com/wiki/index.php/File_Format_MXD