As per the GDPR, personal data is defined as any information relating to an identified or identifiable individual, whereby information that could be used on its own or in combination with other available data to identify an individual is covered. Thus the scope of the definition of personal data is extremely broad and could potentially cover a wide variety of data.
Under this definition not merely data commonly deemed as personal in nature will be included (e.g. names, physical addressees, email adresses), but in addition also data such as IP-adresses, behavioral data, location data, financial data, and much more. Hence for Prezentor customers, at least a majority of the data you collect on contacts will fall within the definition of personal data. One should also be aware that even personal data which has been rendered immediately un-referable to an individual (pseudonymized) can be considered personal data if it is possible to link the data to the individual by auxiliary means.
See also: Changes in obligations due to GDPR