Herbarium and Museum data have not all been digitised let alone their locations made into spatial data as managed by a Geographical Information System which we will introduce fully sometime latter.
For data to be mapped in a spatial system it needs to have co-ordinates defined by X and Y axes.
The most common method (but certainly not the only one) is to use Geographical co-ordinates. When representing location on the earth's surface this is is done using Degrees Minutes and Seconds as units of measurement.
Geographic coordinate system http://en.wikipedia.org/wiki/Geographic_coordinate_system
Decimal Degrees System http://en.wikipedia.org/wiki/Decimal_degrees
Different degree formats: Resolutions and conversions http://home.online.no/~sigurdhu/Deg_formats.htm
Even though data collections and data sets may have spatial data this data may still only be in table form. This data is referred to "attribute data" and needs to be linked to "objects" which represent spatial phenomena. This process is called spatially enabling data and is basically the same as plotting real data on a an X Y scatter graph.
Attribute data may also have location data capture in the form of a place name, a zip code or a street address. You can using an existing GIS system which has these objects and an attribute table and Geocode that table so it becomes spatially referenced and viewable in a GIS.
Please read this article on Geocoding http://en.wikipedia.org/wiki/Geocoding