Many people are not aware of parameters in Inventor but they utilize these gems every day of their Inventor lives. Each dimension you enter for both features and sketches makes use of parameters.
The trick to parameters is to make it legible to you while working with them. This is where renaming your parameters come in. It is easy not to rename them but when you are trying to change a design by using the parameters d1,d2,d110 these parameter names do not make for easy interrogation.
If you rename them to something more legible like Length, Width and Height it is much easier to identify what you are looking at.
To reuse parameters in different designs you can create them and then save them in the template format. This way they will be available whenever you start a new part file or assembly file.
If you need to control aspects of your new design with values (dimensions) in an existing model you can link the parameters from there. This is also called skeletal modeling.
If you open the parameters dialog box you will notice there is a link button in the bottom left corner. Selecting this will allow you to browse for a file (either ipt or iam) where you will be able to select parameters that have been utilized. These parameters that sit in your external file, then control aspects of your new design once assigned. As shown above, these parameter values can either be accessed from your Inventor files or from an excel spreadsheet.