The eIDAS Regulation defines a qualified electronic signature in Article 3 (12) as a guaranteed electronic signature that is created by a qualified means designed to create electronic signatures and that is based on a qualified certificate for electronic signatures.
It has got the same validity as a handwritten signature.
A qualified certificate is defined in Article 3 (15) of the eIDAS as an electronic signature certificate issued by a qualified trust service provider creating trusted certificates and fulfilling the requirements set out in Annex I to that Regulation.
Technically speaking, a qualified certificate is same as any conventional certificate.