Heterodont : It is the condition of having different types of teeth. For example humans have four different types of teeth namely incisors, canines, molars and premolars.
Thecodont : The type of attachment in which each tooth in embedded in a socket.
Diphyodont : The type of dentition characterised by two sets of teeth. First set is of temporary and second is of permanent.
Diphyodont is the condition of having two set of teeth during the life of an organism. For example : humans are diphyodont as they have two set of teeth namely temporary milk or deciduous teeth which are replaced by permanent or adult teeth.