'Parameteric Modeling', as it pertains to CAD systems, generally is referring to the ability for gormetric models (generally solid bodies, but this can also apply to surface models as well as 2D profiles, oftern referred to as 'sketches') to retain numerical values as part of the model definition which can be edited to cause the model to update. And example might be to define a cylindrical solid model by having values for the Diameter and the Length as well as a location for it's origin and even the direction of the axis of the cylinder.
Another example might be what are called 'Features' such as a 'Blend' or 'Round', applied to an edge of a solid or between two sheet bodies or surfaces, where the radius value is retained, or a hole where you have a Diameter and perhaps a depth if it's not a thru-hole. A Chamfer, might be a simple size if it'e a 45° Chamfer or a Size and Angle or two offset dimensions.
Now these values or 'Parameters' can generally be either single valued numbers or algebraic formulas, sometimes called 'Expressions'. To provide maximum usability there's usually some sort of formula builder or Expression editor which allows you to create any number of expressions to solve mathematical problems any of which can be used as a 'parameter' to drive one or more 'features' or size of a model.
Anyway, that should get you started.
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
UG/NX Museum:
To an Engineer, the glass is twice as big as it needs to be.