I have read in several of the technical documents that it is possible to modify and/or extend the data model within Salesforce.com, however I have not been able to find the technical details of how one does this. I would like to understand what is involved in the following:

1) Removing existing properties of data objects (is this possible?)

2) Adding new properties to data objects

3) Modifying the relationships between data objects (if possible)

Thanks in advance for any help that can be provided.