As new information is entered into the software from online forms the software is able to compare that information against that of existing constituent's within your account. The default comparison uses First Name, Last Name and Email to confirm a match. Each of these three fields must match exactly what is listed on the existing Constituent's record. If all three are a match then the newly submitted data will be inserted into that existing Constituent's record. In cases where there is not an exact match a new Constituent record will be created.
Within your Account Settings you are able to modify which fields are used for this match. Specifically you can remove First Name, Last Name or both from the matching criteria. The could then allow for only a Email to be used to match or only half the Name and Email. You are unable to remove Email as one of the matching Criteria.