What is the difference between clients and contacts in D-Tools Cloud?

Clients are customers or who you're doing work for. Contacts are individuals you may contact when performing a service for a particular client. A client can have multiple contacts associated with it and a contact can be associated with one or many different clients. Clients and contacts have a parent and child relationship.

You may be working with ABC theaters and installing speakers at two of their locations. The client would be "ABC Theaters" and the contacts assigned to that account could be a manager from each location you may need to contact while performing the work. 

You may have a couple of clients for which you perform similar tasks/services and use the same builder. You may have an account with Acme, Zoogle, and NCG for example. Since you use the same builder, you can add this contact to all three clients and have this person's information populate in the Opportunities/Projects going forward.